Among his colleagues on the professional tennis circuit, who acknowledge that he is a player of rapidly maturing skills and slowly maturing temperament, 20-year-old John McEnroe is almost universally known as "Junior."

But in Great Britain, where sportsmanship and civilized manners remain as much a measure of champions as their results on court, the gifted left-hander from Douglaston, N.Y., recently has acquired a less affectionate nickname: "Superbrat."

By far the most interesting aspect this year of the media blitz that annually precedes the Wimbledon championships, which begin Monday at the All England Lawn Tennis and Croquet Club, has been the protracted newspaper "trial" of McEnroe. He clearly has supplanted the tiresome and injured ilie Nastase in tabloid headlines as "The man the Centre Court crowds love to hate."

The British press and public admire McEnroe's rare and exquisite talent-his talent - his athleticism, racket control and extraordinary "feel" for the ball and the things he alone can make it do.

But they find it difficult to accept his scowling competitiveness and sometimes churlish behavior with stiff upper lips. To them, he is less "Junior" than "juvenile."

The British prefer their champions to be gallant but good-natured fighters. This is why Evonne Goolagong Cawley, the smiling and ever-sunny Australian, will be an overshelming sentimental favorite to foil the fiery defending champion, Martina Navratilova, and those impassive American challengers.

The top men have no equivalent to Goolagong, who makes even the tensest match seem like a cheerful Sunday afternoon frolic in the park.

The British audiences prefer the icy, undemonstrative stoicism of Bjorn Borg, the machine-like Swede who is seeking his fourth consecutive singles title, to the hot-blooded tempestuousness of McEnroe and Jimmy Connors, the champion of 1976 and runner-up to Borg the past two years.

At Wimbledon, remember, the main events are still referred to quaintly as the "gentlemen's and ladies' singles."

Few knowledgeable observers question that McEnroe has the ability and determination to win tennis' oldest and most prestigious title, but many wonder if he will ever be a gentleman.

The doubts poured forth savagely in the public prints here recently when McEnroe misbehaved - albeit $&(WORD ILLEGIBLE moderately, by contemporary standards - at a tournament at London's stodgy Queen's Club.

McEnroe's abrasiveness, self-critical sulking and rabbit-eared sensitivity have created enough scenes, and brought him enough scoldings, to make him accustomed to criticism by now, but even he was startled at the lashing he took in the London dailies.

"Ban the Superbrat," screamed the front page headline in the mass-circulation Daily Mirror, beneath a photo of a snarling McEnroe, after he had been docked a penalty point for sassing an umpire for the second successive day.

"Shut up, Mac," advised the Daily Mail, which changed its back-page headline in later eiditons to the more eloquent: "McTantrum."

"Kick him out!" trumpeted the Daily Express, another popular tabloid, amplifying the assertion of Sandy Mayer, McEnroe's angry quarterfinal victim, who siad that penalty points and fines are no deterrent to the millionaire manchildren of tennis, and that disqualification from tournaments would be the only effective means of discipline.

McEnroe did not fare much better in the reports of his "incident-marred" victories than he did in the hysterical headlines.

"He is the most vain, ill-tempered, petulant loudmouth that the game of tennis has ever known," declared The Sun, never known for its subtlety.

"The only question is do we want a Wimbledon champion like McEnroe, who violates every rule of sportsmanship, what used to be called good manners and the whole spirit of the game?" wondered Daily Mirror columnist Peter Wilson, whose byline proclaims him to be "The man they can't gag."

"...McEnroe is both a bore and a boor. I do not pretend to be a psychiatrist, but I do know no one is greater than the game," Wilson continued.

"And if McEnroe doesn't, or can't, or won't mend his ways, to become something approaching a civilized human being instead of only a brilliant, ill-mannered, primping prima donna of a player, it might well be necessary to adopt Mayer's suggestion of banning him from tournament after tournament.

"No one who understands lawn tennis can doubt for a moment that he is a genius at the game. Equally, no one who loves lawn tennis can tolerate McEnroe's hobbledehoy antics much longer."

Even the more reserved and serious of the London papers took McEnroe to task on this occasion.

The weighty, conservative Daily Telegraph duly noted that "He seems intent on showing a rasping personality on court," while the more lively Guardian lectured him sternly:

"When McEnroe was rising from the ranks of the obscure there were those who went to some lengths to excuse his sullen face, short temper and vicious tongue He was still a boy in a man's world, unaccustomed to the pressures, the rewards or the glamor. But the time for excuses was over, McEnroe is, by common consent, one of the game's finest players. He has beaten Borg and Connors - the two men ranked above him - he has a a responsibility to his position and must face it."

Neutral observers - especially those who realize what the British find hard to comprehend: That McEnroe's deportment has improved considerably over what it was a year ago - agreed that the newspaper reaction to the Queen's Club incidents was a classic case of overkill.

"It was blown out of all proportion," said Frank Smith, the Grand Prix Supervisor responsible for enforcing the players' code of conduct at the tournament. "If Carter and Brezhnev had come to blows in Vienna, it couldn't have made a bigger impact in the British papers than this did."

McEnroe - who admitted afterward that he had been wrong to tell an umpire, "I'll make sure you're never on my court again" - shrugged off the whole episode with reasonably good humor.

"I don't know whether or not you guys write the headlines," he told the assembled British tennis press after beating Victor Pecci in the final, a match in which he behaved admirably, "but over here, if you say anything during a tennis match, they make it seem like you committed a murder or something."

He refuted Mayer's charge that his habit of constantly retying his shoelaces was a stalling tactic, calculated to disrupt an opponent when he is about to service a big point.

"I'm not trying to bother the other guy. I'm just a fidgety person," McEnroe said. "ti like everything to be perfect, and my shoelaces tight."

He also vowed to be on his best behavior for Wimbledon, for purely pragmatic reasons: "I've learned that it doesn't do any good to be negative to umpires and linesmen.It only makes them negative to you."

"Sure, I'd like to have the crowd on my side, but I'm not sure how to do it," he added "I don't know if the English will ever be on my side, because I have a reputation now that's hard to change." $ tOne of the

One of the things that makes McEnroe so difficult to swallow is that even when he hits a glorious winner, he looks unhappy. He never is satisfied, and never smiles. He makes tennis look less like a joyous game than some kind of unspeakable torture.

He has never understood that audiences do not appreciate agony in the face of one so young and so gifted. So he points and grimaces, kicks the court and curses under his breath, complains and twists what has been aptly described as his "Irish pug face" into a thousand self-critical expressions.

"That is just me," he says. "The faces are part of my personality. If I miss a shot, I make a face, even in practice."

It is all part of his perfectionist, fiercely competitive nature - Mayer calls it his "enormous ego" - and McEnroe evidently feels it is more important to be himself than to try to win friends and influence galleries.

He is confident in the abilities that have swiftly carried him to No. 3 in the computerized world rankings of the Association of Tennis Professionals, behind only Borg and Connors. (McEnroe is seeded No. 2 at Wimbledon, behind Borg and ahead of Connors, because it was uncertain until last Tuesday whether Connors would play or withdraw to be with his wife, who is expecting their first child momentarily.)

McEnroe, who led the U.S. to its first Davis Cup triumph in five years last December and won the Grand Prix Masters over a depleted field in January, beat both Connors and Borg to win the World Championship Tennis finals at Dallas last month, his most satisfying victory to date.

But is he ready to win Wimbledon? He thinks he can, and so do many experts. The biggest questions that remain are, is Wimbledon ready for "Superbrat"? and if he does win, will the British give in and call him "Junios?" CAPTION: Picture 1, John McEnroe's tantrums are big news in Britain. "I don't know whether you guys write the headlines," McEnroe told reporters, "but over here, if you say anything during a tennis match, they make it seem like you committed a murder or something." By Harold Hoover - The Washington Post; Picture 2, Chris Evert chases a shot during her three-set victory over Martina Navratilova in final of Eastbourne (England) tournament. AP