The camouflage-suited, porcupine-haired figure sprinted down a crowded concourse at Chicago's O'Hare Airport, his sunglasses glinting, a cigar in his mouth, a precious cargo held gingerly before him. At the gate he was ordered to stop. He paused, considered his options, then chucked his plastic cup of beer into a convenient trash can and dashed onto the waiting airplane.

Fidel Castro? A thirsty Contra? A visitor from Mad Max: Beyond Thunderdome?

No, it was Jim (Mad Mac) McMahon, the world's first rock 'n' roll quarterback. The Man From Punk was kicking off the Chicago Bears' part of the NFL's invasion of Britain, a brilliant bit of corporate and cultural promotion that peaked a week later on Sunday when the Bears beat the Dallas Cowboys 17-6 at sold-out Wembley Stadium in London.

There have been other NFL exhibitions in foreign lands—Japan in 1975, Mexico City in 1978, London in 1983—but no other that was promoted by the league or carried the upbeat possibilities of the '86 extravaganza. International TV rights? Product marketing? European expansion? The possibilities lie there like so many seeds waiting to be tended by the NFL's green thumb.

You want to think really big? Well, how about worldwide expansion of the league? How about the London Rippers versus the Tokyo Kamikazes in Super Bowl MM, played on a spaceship orbiting the moon? According to Ralph Miller, the sales and promotions manager of Wembley Stadium, a quarter of a million tickets could have been sold to Sunday's game alone. Miller wants the NFL back in London for five preseason games next year. ''And all this was done naturally,'' marveled the Cowboys' president, Tex Schramm. ''It wasn't forced on the English. For some reason they were just ready for it.''

Certainly they were anxiously awaiting McMahon, who, once aboard the 747, joined the other half of America's weirdest sports duet. That could only be William Perry, the almighty Fridge, the Bears' cuddly defensive tackle who reportedly earned $3 million—and another nickname, the Endorser—in the past year for being friendly and large.

It was nice that the Cowboys and all the other Bears came to London, too, but with these two, they weren't really needed. (''Are you as good as the Refrigerator?'' an employee at Hyde Park's Intercontinental Hotel asked Walter Payton when the team arrived.) In the combined personae of Mad Mac and Fridge lay every quality the English expected to find in what must now be called ''American football''—wealth, talent, controversy, cheekiness, girth and odd clothes.

Check that. The Cowboy cheerleaders were needed as well. They arrived on Thursday and were glowingly described by London's Daily Express as ''three dozen lovelies with high IQs plus a formidable knowledge of current affairs.'' For the Cowboy players, who came all the way from training camp in Thousand Oaks, Calif., eight time zones from London, the main souvenir they would take back home with them was jet lag. ''I never did get straight,'' said Tony Dorsett. ''I spent most of my days sleeping and went to discos at night.'' Indeed, such traditional training-camp hardships as curfews scarcely existed for either team in London. ''This is a cultural exchange as much as anything. We want the players to get out and meet people,'' said Bears G.M. Jerry Vainisi. One of the first people Bears tackle Keith Van Horne met was a punked-out lad who posed for a photo with him in Piccadilly Circus. ''Mind if my friend joins us?'' asked the youth, who then fished a spotted rat out from somewhere in his clothes and stuck the live rodent in his mouth.

English culture was alternately confusing and amusing to the players all week. It was as hard for them to fathom pubs closing at three in the afternoon as it was for the Brits to understand the appeal of living in a 20-story high rise without a garden. Some English words also created mild problems, gridiron becoming ''pitch'' and teams becoming ''sides.'' ''How long does it take to do your hair, luv?'' Bears kicker Kevin Butler asked a spike-coiffed girl in Sloane Square one afternoon. The girl, whose studded leather jacket bore a drawing of the late ultrapunker Sid Vicious's coffin, was unwilling, or unable, to answer.

Yet it was obvious to all that the contrast between the cultures was exactly what was at the root of the American football craze in Britain. ''This country is too set in its way,'' said shaved-headed Jerry (the Animal) White, who plays for the Streatham All Powerful Olympians American football club, while watching the Cowboys and Bears practice at Crystal Palace on Tuesday. ''England needs a new sport,'' said the Animal, whose two-year-old son, Eddie, romped at the end of a leash held by dad. White then added the requisite axiom of any discussion about the sudden popularity of American football: It is ''refreshing'' to soccer-saturated Brits because ''the violence is on the field, not in the stands.

''It takes some tensions outta me system, and it's legal,'' concluded the Animal. This observation was followed by much nodding from his All Powerful Olympian teammates, Nigel (the Tank) Myles and Tony (Buggsy) Brock. The Tank is a Perryesque chap of ''22 stone'' (308 pounds), but where did Buggsy get his nickname? Brock removed his two front teeth. ''A naughty tackle,'' he grinned gappingly.

Some of the NFL players didn't care much about their ambassadorial roles or cultural possibilities. After all, this was also an extra preseason game, one for which they would receive little money but a great deal of disorientation. ''If England was so nice, why did everybody leave and come to America?'' Cowboy tight end Doug Cosbie wanted to know.

The Fridge, mobbed wherever he went, tried to stay open-minded while laying down certain socio-ethical limits. ''You'll never see me in no punk- rock outfit with no rat hangin' from my mouth,'' he stated vigorously.

Very rapidly, however, he found himself sucked into the vortex of the British tabloid whirlpool. The daily newspapers went after ''Bill'' Perry the way wolves go after a bunny. On Tuesday one such paper ran his picture on the same page as the headline: CROSSBOW KILLER ON THE RAMPAGE. On Wednesday, in a story entitled FEARLESS FRIDGE, the Daily Express wrote: ''Legend has it that in his 28-stone college days, he once ate five chickens at one sitting, then jogged down to McDonald's for $55 worth of burgers.'' Another paper called him: ''The biggest piece of lard to make his name in sports'' and ''the ugliest athlete on earth.'' On Saturday came the headline in The Sun: FRIDGE IS RED HOT IN BED, SAYS WIFE, followed by the Sunday Mirror's: THE WEMBLEY COWBOY'S MESSAGE TO THE 'FRIDGE': KISS MY FEET.

The latter was an alleged quote from Dallas's Ed (Too Tall) Jones, who laughed when confronted with the news. ''I haven't talked to the press at all,'' he said. ''You know I could come up with something a lot better than that—or worse—if I wanted to.''

Players learned to chuckle at the silliness of the ''comics,'' as the tabloids are sometimes known in England, even to understand that a certain amount of myth-making was a necessary thread in the fabric of this game, particularly in a country that fancies cricket and snooker as major sports. Good thing. AMERICA'S GIANT SUPER BOWL CHAMPIONS—THE CHICAGO BEARS—JET HOME TODAY LEAVING A TRAIL OF HEARTBROKEN GIRLS BEHIND, leered The People on game day, using as evidence an unnamed hotel barman's claim that ''one muscle-bound Bear'' bought champagne for ''eight stunning girls'' and then took them all to his room.

On Friday, Bears fullback Matt Suhey, an off-season commodities trader, visited his friend and fellow options trader Andrew Coulton at the Goldman Sachs Futures Limited trading offices in Old Bailey. Coulton, too, has become an American football fan, and the Englishman tried to explain how magical the $ notion of size is for a country that has grown accustomed to its own declining economic and political influence. ''You do get big lads here, but they're plodders, not athletes,'' Coulton said. ''We're fascinated by the skill level in the NFL, by the uniforms, by the modern stadiums in America. We see the show Dallas on TV, we see everything on such a big scale—big crowds, big money, big style—and it affects us. It's glamorous. In a way, we still think all Americans are millionaires.''

Such big shots obviously needed protection, and Scotland Yard, in conjunction with NFL security, the U.S. State Department and the FBI, gave it, the only breakdown being a streaker who made it onto the field during Sunday's game. But there were no violent incidents of any kind during the two teams' stay in London, except for an old-fashioned skirmish between two Bears rookies in practice and the expected exchanges of ill will between the Bears and Cowboys during the game. ''I think I just don't like Texans,'' said Van Horne, trying to pinpoint the problem.

After the planes touched down at Heathrow, and the famous detective spaniel, Oscar, had sniffed everyone's luggage and found no drugs or explosives, both teams and their parties—more than 200 people on each side—seemed to forget about earlier worries of terrorist retaliation for the U.S. bombing of Libya last April. But not everyone did. An unattended briefcase was carefully and unobtrusively removed from the Wembley press box by a guard during the game, but it was found to contain nothing but ''papers.''

Mostly everyone had a jolly good time. The Fridge took a rambling excursion through London one day, firing his camera wildly from the back seat of a car while its hired driver snaked through streets as narrow as footpaths. ''I got Big Ben,'' Fridge shouted, shooting through the window. He spotted the pond in St. James's Park. ''Can you fish there?'' he asked. ''When I'm done with football I'm going back to Aiken, and I'm going to fish, fish, fish and fish.''

The car passed a statue of Winston Churchill and Fridge mused, ''Now he was a big man.''

When the car parked, a not-yet-teenager came up, knocked on a window and asked Fridge if he would step out to be photographed. Fridge declined. The boy pleaded, ''I could make a lot of money.''

After the youngster left, the Fridge shrugged and said, ''See? Nowadays all kids think about is money. When I was a kid all I thought about was eatin'.'' In short order the Fridge cheered up, and as the car rolled onward he said to no one in particular, ''Yes, this is a bloody good town.''

All Cowboy player personnel director Gil Brandt could think about was the new vistas for talent opened up by this trip to the edge of Europe. ''I'd love to get the first big Russian center,'' he said wistfully. Other Cowboys were not so single-minded. One night, Cowboy linebackers Jeff Rohrer and Steve DeOssie took in A Midsummer-Night's Dream at the open-air theater in Regent's Park—no stage gimmicks, no hidden microphones, just good stage actors doing the master's work. The two Cowboys almost had apoplexy from laughing so hard. ''We were howling and stamping our feet,'' said Rohrer, a former Yalie, who went back the following night, alone, to see the play again. ''The actors project, they enunciate. One character insults another, calls him 'this spotted and inconstant man.' What language. I can't complain about this trip at all.''

Doing their own scouting, the Bears signed an English kicker, one Russell Wilsmer of Isleworth, Middlesex, on Saturday and let him boot their first kickoff. It was not a good kick, and one could almost hear Wilsmer's mates back in Isleworth pubs sighing in distress.

It rained off and on throughout the game, but the 82,699 fans didn't seem to mind. The Fridge scored a touchdown, played a little linebacker, and the Bears won the sloppy, meaningless scrimmage.

By most accounts the tour had been a fabulous success. The NFL will do no more than break even financially, but that, said league administrative director Joe Rhein, is more than enough. The beachhead has been established.

Outside Wembley, in the evening mist, 13-year-old Gavin Williams of London walked through the parking lot with his father, Reggie, a postal worker. Young Williams was ecstatic. He passed up a two-week vacation with his uncle in Spain just to see this contest. He loved everything about the game, ''except,'' he said, ''the in-betweens are too long.'' But those pauses, he already knew, were for television timeouts, the lifeblood of the NFL, the very sap of American electronic enterprise.

So did he regret not seeing sunny Spain? Not one bit, he said. ''You know,'' he explained in a knowing voice, ''this might not happen again.''

Gavin, old mate, it will. Trust us, it will.