From a land long regarded as the bellwether of western culture, the saga of Britain's commercial breakfast television is a worthy morality tale.

With the help of a nasal-voiced puppet named Roland the Rat; a host who says he got the job because "he is so very ordinary," and Diana Dors, a corpulent ex-movie star who has been shedding pounds on the air, TV-AM has been transformed this summer from one of the great misfires of broadcasting to a smashing success.

Some months ago, TV-AM was making little headway against the British Broadcasting Corporation's relatively sedate version of early-morning television. At one point, the BBC program had about 10 times more viewers at peak segments than did TV-AM.

But now as Roland the Rat takes to the airwaves each morning, TV-AM is pulling ahead at a ratio of 6 to 2.

"This may be the first time in history that a rat has come to the aid of a sinking ship," a BBC spokesman said.

The key to the new success at the commercial station is fundamental--it has abandoned the high and mighty for the comfortably, even embarrassingly, humble.

When TV-AM went on the air Feb. 1, it boasted the glitz and glamor of some of the biggest names in British television, and had, according to its chairman, former British ambassador to the United States Peter Jay, "a mission to explain."

Among the early features was a lengthy piece on a blue-ribbon panel's study of north-south economic relations.

The guests on that particular waking hour were a dour professor from the London School of Economics, whose views were incomprehensible to anyone who had not read the study, and former prime minister Edward Heath, who wore a cardigan intended, no doubt, to make him seem more jocular. The questioner was David Frost, known to Americans for his gripping--and lengthy--post-Watergate interviews with Richard M. Nixon. But on this occasion, even Frost was plainly, if not altogether surprisingly, stupefied by the program.

So, apparently, was the audience, and within six weeks Jay was ousted. Two of the celebrated on-air personalities, Angela Rippon and Anna Ford, also were fired. (Ford was back in the limelight briefly in June, when, at a swank cocktail party, she hurled a glass of white wine at the executive who fired her.) Frost was effectively downgraded to cameo appearances, and he went on vacation.

The company's finances were as messy as its programming, because of the poor ratings and an unrelated, interminable dispute over fees for actors in commercials.

"We were five weeks from bankruptcy,"said Greg Dyke, the 35-year-old producer of a breezy early-evening magazine show who was brought in by the new management in a desperate salvaging mission.

Dyke's formula was, as he puts it, "populist." As his principal hosts he chose two young unknowns. One was Nick Owen, who was promoted from sportscaster and who explained he had been selected "because I'm so very ordinary really." The other, brought in from a local show in the hinterlands, was Anne Diamond, whose buoyant freshness was the opposite of Rippon's and Ford's haughty cool.

The show's weatherman, a sturdy ex-military officer named Philpott, was replaced by another attractive post-adolescent from the provinces.

Out went virtually all heavy news, leaving what amounted to a headline service. The diplomatic correspondent was fired along with, said Dyke, "anyone asking over 18,000 (about $27,000) who wasn't truly worth it." As it cut back on foreign news, the station also stopped taking regular feeds from ABC.

"We were covering Lebanon as though it was daily soap opera," Dyke said.

There were occasional interviews with politicians, and during the general election in June, Dyke used veteran journalist Robert Kee, one of the original investors in TV-AM, whose series about Ireland was a hit on PBS last year. But the new tone was decidedly and unashamedly down-market.

Diana Dors, who once rivaled Jayne Mansfield as a big screen sex symbol, became the symbol for every overweight woman in the British Isles as she began an effort to lose 50 pounds. To date, according to her latest tip of the scales, she has dropped about 40.

Ratings started to improve. Dyke noticed they took an extra jump on a week when children were home from school. So as vacations arrived, he decided to go all out for the kiddies, hoping to corral their parents, especially their mothers, along the way. One result was Roland the Rat, who, with his companion Kevin the Gerbil, introduces an extended cartoon segment.

In addition, eight times a morning the cameras switch to a British beach resort for such scenes as the mayor of Blackpool dancing with a man in a gorilla suit ("great theater" in Dyke's view), the gyrations of a belly dancer, a bit from a comedian called Jiminy Cricket or an interview with monosyllabic tots about their favorite holiday spots. It was these innovations that sent ratings soaring, confounding, Dyke said with some pride, the usual trend toward smaller television audiences in the summer.

Dyke, who is uncannily like his description of TV-AM's programming philosophy--"sharp, short and fast"--dismisses suggestions that he is pandering to the public. "When Mr. Dyke rode to the rescue," the Times of London sniffed this week, "there were fears he would take an exclusively low road to recovery. This he has done."

"That's fine for the Times to say," Dyke replied. "It loses millions and would die overnight if it wasn't for the Sun," a tabloid with the same owner that features bare-breasted women and has Britain's largest daily circulation. "The assumption of serious journalists is that being popular is easy," Dyke added. "Well, they're wrong."

Nonetheless, Dyke promises that come fall, the program will have at least one five-minute segment a day, at 7:10 a.m., aimed at serious news, usually involving an interview. The adjoining segment also will be newsy, he said, but at 7:25 a.m, the mood will shift abruptly--to an installment of a Popeye cartoon serial. There will also be bingo, pets, hobbies and Dors, who will stay in an as-yet-undefined role.

And David Frost, proving he's a trouper at heart, has accepted the relative obscurity of an eight-week Sunday morning stint, now the least-watched of TV-AM's programs.

Roland the Rat's future, however, is in doubt. His weekday segment will probably end when school starts (and Dyke expects ratings to dip) but he may well turn up elsewhere. A new problem is the disclosure this week that Roland's creator, David Claridge, has a second life--proprietor of a London sex club.

Observed Claridge: "It was all very embarrassing."