IT IS ENTIRELY plausible to believe that John Reed is a figment of W.S. Gilbert's implausible imagination.

And John Reed, O.B.E., aka: Sir Joseph Porter, K.C.B., Ko-Ko, Jack Point, John Wellington Wells, the Lord Chancellor, Major General Stanley, King Gama, the Duke of Plaza-hf-Toro, Reginald Bunthorne, Robin Oakapple and one or two others, the designated "principal comedian" of the D'Oyly Carte Opera Company, does nothing at all to dispel the illusion.

Indeed, sipping at a Scotch and ginger ale (warm) and nibbling at a plate of sausage and cheeses (cold), he says, at various times, of Jack Point (Yoeman of the Guard). "He's so me ." Of Ko-Ko (Mikado): "He's so me ." And of Bunthorne (Patience): "He's so me ."

And so he is essentially indistinguishable from the characters he has played for some 26 years now, this slight, mercurial, affable, sentimental, leprechaun of a man with his only-just-beginning-to-gray, only-just-beginning-to-thin curly red hair, his deep blue eyes, his chuckle and his sigh and his use of "dear" almost as a comma in a very near nonstop patter song of an interview.

"Yes," he concedes, his eyes twinkling - they twinkle a lot, even behind the glasses he wears offstage - "I'm the daddy of them all," the veteran of the company, for the past quarter-century or so the acknowledged master of the uniquely Gilbert and Sullivan patter song, the definitive interpreter of the tragi-comic, hilariously funny, painfully human, outrageous, forlorn and infinitely lovable characters who wend their singular ways through the operas of Gilbert and Sullivan.

"Oh, I'm very nervous, dear," he says when he's asked how he keeps these characters fresh night after night after night. Then he says, very fast:

"Ooh, yes, I'm very highly strung. I'm nervous. It's not pleasant for me but I can't help it. After all this time I just take it. It always comes back to just one word: Caring. I really care. I care for my own pride in the performance, I suppose, and I care for the audience. All I want to do is please people so I do my utmost for them. Really, that's what I do it for. I do encores until I drop if they want me to." An Accident

It was all something of an accident, the extraordinary union of John Reed and the D'Oyly Carte Opera Company. "I had no idea what this D'Oyly Carte thing was at all," says Reed. He'd been acting and producing in a repertory company - just straight acting, no musicals. A friend heard the D'Oyly Carte was looking for an understudy-chorister and told him about it. "Learn the Nightmare Song (lolanthe)," the friend had advised.

"Well," recalls Reed, "memorizing didn't seem much for someone in a repertory company," and that, to all intents and purposes, was that, to all intents and purposes, was that. He hadn't even seen one of the productions. "So I thought I'd better go along and see what this D'Oyly Carte is, anyway," and he saw "The Gondoliers."

When he learned he'd won the part, "in a weak moment," he recalls, "I said yes. If I'd stopped to think I'd 'ave said, 'I can't do that. What am I doing going into an opera company? I'm not a singer.' I never professed to be a singer.

"Yes, but dancing. I've always danced ever since I could remember. Tap and Ballroom - I have medals for ballroom - and a slight bit of ballet. It always stood me in good stead because for these particular parts you don't have to be the world's best actor or the world's best dancer. But you have to have an accumulation of the whole thing."

He paused. Sipped at his warm Scotch. "Above all," (twinkling again) "above all, you must be dapper. And small. If you're that, and you've got the other things slightly, well then you have the finished article."

John Reed is dapper, certainly, and small (5 feet 7) and though he complains that he's finding his costumes are getting a bit snug, his acrobatics and antics on stage belie any "slightness" to his dancing skills, and if he doesn't admit to the professionalism of his singing now, he's the only one, and, of course, his acting tops off the "accumulation" that could make him the Gilbert and Sullivan lead of all time.

"But," he sighs, wishing, he says, that all the old performances could have been "in the can" like old films, "while you're living, dear, you're never as good as the old ones. Not until I leave will anybody think that we've really done anything." 'Good Luck, Son'

Reed grew up in the north of England, county Durham, where his father was a butcher ("they slaughtered their own in those days, you know"). His parents and one of his three sisters have died, and he has never married. He is devoted to his two sisters, but the company is his family as well.

He's always had "a mum" in the company, ever since a contralto in the chorus told him she thought he'd been "so cute" when, as understudy, he'd gone on for an ailing Ko-Ko. "You were so cute I wanted to mother you," she told him. "So I said 'be my mum' and that's what she became," says Reed. "And it's been handed down - when she left - it had to be a contralto - and somebody else took over. For the past 16 years or so it's been Beti Lloyd-Jones. Never a night, not one night since then that she doesn't come to my dressing room and kiss me and say, 'Good luck, son."

"That's what I mean," he says softly, "about the family thing." Savoring the Feeling

Reed was pleased with the Kennedy Center audience reaction to the Lord Chancellor's Nightmare Song the other night, and, during the interview, just after the performance of Iolanthe, he was still savoring it. (Performances of "Mikado," "Pirates of Penzance," "Princes Ida" and "Pinafore" fill out the company's four-week stay here.)

"Sometimes you can't feel the reaction," he says. "In some theaters it doesn't come up to you, but tonight I felt it. What is so very pleasant is they're laughing at different places, mainly, and a little more. Perhaps it's because English audiences are used to it, know what's coming next.

"Tonight," he says gleefully, "they were taking little points out of the songs . . ."

Reed has little trouble with the patter songs, he says. ("Oh, touch wood! Mind you each time I say this, it could happen tomorrow night.") But he concedes some are "harder to get your tongue around than others." "Nightmare," he notes, has a kind of continuity, a sort of story line. A killer is John Wellington Wells' song from "The Sorcerer." "Just a list of words" notes Reed, "now that is difficult. Nothing leads you to the next one. You could put them in any shape or form if you lost the rhythm," and then, in perfect rhythm, faster than a blink, he breaks into "changes organity/with an urbanity/full of Satanity/vexes humanity" and "barring tautology/in demonology/'lectro biology/mystic nosology/sprit philology/high-class astrology.

"It's a strange thing," says Reed, "I suppose after all this time, because my brain goes like this, I can be singing the 'Nightmare Song' and knowing what I'm singing, and at the same time I'll think, 'Oh, that poor woman in the audience, she can't see a thing because that woman's hat in front is too big.' That's what goes through my mind while I'm singing the song."

Reed was honored by the queen last year. But, he acknowledges, it was "a bit of a disappointment."

"I went to the palace, of course, (for his Order of the British Empire) and I suppose it was all very pleasant. Everybody was dressed up and I was done up like a penguin. The uniforms were there. She's surrounded (Queen Elizabeth) by yeomen of the guard and she has this little, tiny simple blue dress on, a short blue dress, no jewelry. And I would like to have seen a tiara, I think, or something." He signs.

Traditional. But not so traditional that he won't keep a bit of creeping American TV commercial out of a production.

"We're amused by commercials on television. Last time we were here it was 'shyke and byke and ay'll hayelp.'"

"'Sha-yke' and 'Ba-yke' and 'Hayelp,'" he repeats. "Something like that gets around the company."

So much so that it's made it into "Gondoliers." "Do you have any sha-yke and ba-yke?" asks the Duke of Plaza-Toro (Reed) of a waiter.

Well, it breaks up the company, anyway.