COMES NOW THE Worst of Families.

"I, Claudius," a 13-part BBC import premiering tonight on public television, is the "Upstairs, Upstairs" of seamy Caesarian Rome, a fascinating and elaborate tale - told by a man once thought to be an idiot - that proves again British superiority at historical, pedestal television.

It is not the kind of television all television should be, since it is the heavily verbal, theatrically rooted and extravagantly rather than intimately performed. And yet, though it may actually defy basic properties and capabilities of the medium, this latest import to march under the "Masterpiece Theater" banner proves immensely, compulsively watchable.

It's so good it's a little embarrassing, since a current attempt by American public TV to imitate the British style, "The Best of Families," is proving a lavish calamity. How can it be that a British-made series about ancient Rome, in which almost all the characters are royal personages from the vaults of history, comes across as more pertinent and more creditable than an American-made series about a much more recent American epoch? Part of the answer may be simply that the British have had more practice.

But their method may be as superior as their product. All 13 episodes of "I, Claudius," adapted from the novel by Robert Graves, were written by one person, Jack Pulman, and directed by one person, Herbert Wise. "The Best of Families" is the product of a flock of writers, a coalition of committees and an informal alliance of high-minded funders hell-bent on worth whiling their audience to bits.

Hollywood tried to film the Graves novel in the '30s but the production was colossally ill-fated; the fiasco is chronicled in a documentary that has been shown on public TV and at film festivals. Hollywood and Rome subsequently proved to be made for each other, however - each being communities devoted to the establishment of gods and the exaltation of excess - and so many of our movie-going lives included periodic visits to the land of the Caesars, where all manner of wickedness did flourish, sometimes in color and CinemaScope and usually in a spirit of spectacular and entertaining bad taste.

The Graves novel and the TV version scale Rome and the Romans down to manageable size, and, in so doing, bring a distant and blurry kingdom that was decidedly not Camelot into fresh and immediate focus. True, they murdered, ranted, tyrannized and debauched but, as Darryl F. Zanuck once said of a riotously wicked woman in a 20th Century Fox potboiler, you rather like them. If the series has a great point it is that the intrigues, perversities and injustices of today's world may be subtler than those of old Rome, but they are hardly less pervasive. "I, Claudus" is an amusingly appalling session at a fun-house mirror in the tawdry archives of human history.

It is also handsomely produced, grandly acted and though perhaps slow to start, thoroughly involving in the long run.

In the courts of the four Casars depicted - detailed according to Graves' conceit, in the 80-year recollection of the fourth. Claudius, who died in 54 A.D. - there was enough scandal to keep the National Enquirer presses running round the clock. In fact, for series producer Joan Sullivan at importer WGBH Boston, there was too much scandal. Sullivan took it upon herself to trim some of the shocking scenes in "Claudius" because she personally found them too shocking.

Of course, they are supposed to shock, and the series ran intact on British television without fostering riots or toppling the government. Sullivan says she doesn't care about that. "The British aren't omniscient. They are not sacred," she declares. "They do some really raunchy stuff. They make mistakes. I'm always appalled that while we see the best of British television, they see some of the worst of ours. They love a lot of our really ultraviolent commercial stuff."

Of her deletions, Sullivan says, "I didn't cut any scenes. I cut some shots. I wouldn't want people to think I cut scenes. But some shots I thought on the basis of my judgement of good taste for a mass audience to be indulgent or gratuitous."

Among these was a horrifying scene in Episode Nine in which the mad Caligula, believing himself to be Zeus, stabs the sister with whom he has had an incestuous relationship and not only kills her, but removed from her womb the fet us of an unborn child. And worse, Sullivan removed about a minute's worth of shots from this scene.

In making such expurgations. Sullivan said she was acting in the interests of people in "Albuquerque, Des Moines. Boise" and other cities - people who, she suggeseted, were not as sophisticated as those in places like Boston, New York and Los Angeles. This is certainly an interesting rationale. Other cuts include "one overhead mirror shot that was unnecessary, a wide shot of two nude bodies in bed together" in another episode.

"I don't believe in censorship but I do believe in taste," says Sullivan, echoing the self-defense of censors from before the dawn even of Rome. And then she really gets carried away: "I do have standards," she says, "for my series" - emphasis hers.

One is reminded of two things Boston is famous for bans and beans.

With a preface that will scare off some viewers and, undoubtedly, rivet others to their sets, host Alistair Cooke begins the first episode with a disclaimer. "I think I ought to say at the start," he announces sternly, "that some people are going to more shocked by this series than by most of the dramas that we've shown on "Masterpiece Theater.' But Robert Graves does not make it more cruel or more gamey than the manuscript of Suetonius from which he worked, and Suetonius was there . . .

"On the contrary, Graves has left in only what is essential to the historical plot. Violence is not shown just for titillation and there's no pretense that sexual orgy is some sort of launch pad into a liberating playboy philosophy. Vice is shown for what it is." Cooke also warns of depictions of "incest" and "group sex," but voyeuristic viewers will have to wait awhile for those; the wildest thing in episode one is a brief shot of topless dancers at a feast.

It would be unfortunate, as Sullivan herself says, if the racier aspects of "I, Claudius" were allowed to upstage the "fine qualities" of "a superb dramatic production," each if one may question her qualifications to play moral arbiter for Albuquerque or any other city. As if to vouch for her liberalism, she adds, "There is still a great deal of sexual innuendo and explicit violence that was not removed.

The fine qualities include, as one would expect, the acting. As narrator Claudius at various stages of his life, including the last state from which he wrote his history, Derek Jacobi is both touching and authoritative, and we sense at once the wisdom beneath the stammer, the limp and the congenital clumsiness with which Claudius stumbled througg life. One character says to his grandmother in Episode Five, "That grandson of yours could wreck the Empire just by strolling through it." She herself refers to him contemptuously as "my idiot grandson."

That formidable granny is the dominant presence of the early episodes - Sian Phillips as Livia, second wife of the emperor Augustus (Brian Blessed) and a wicked woman that might have made even the mighty Zanuck quake. She thinks nothing of sending children off to their deaths by poison if they seem to be in the way, or lecturing gladiators with "You're all scum and you know it" before they go into the arena for battle. "I want a good show," she says. "These games are being degraded by the use of professional tricks to stay alive!"

In the opening episode, she has a particularly fine moment during a tiny rampage by the citizenry. "I'm not frightened of that rabble," she snarls, and when splattered with a heaved vegetable she shout out, "You wait 'til my husband gets home!" it's a delicious bravado performance all the way.

Writer Putnam obviously puts into the mouths of these Romans colloquialisms more at home today than in 24 B.C., the first year depicted. The effect is essentially to humanize the character, not to garner cheap laughs in the jarring style of such historical farces as "The Lion in Winter." Yet there is a savory dark-comic tone to this saga, and the unassailable facility of the British actors helps maintain that tone with consistency and dignity. Thus even an incongruous phrase like "goodness had nothing to do with it" or "I wouldn't touch you with a 10-foot pole" doesn't impair the convincing sense of unreachable history brought within our grasp.

More important than that, however, is the justifiably jaundiced viewpoint through which all of it is seen, and this is continuously and inventively invigorating. "I, Claudius" sees dreams of power no less coorrupting than power itself, and these are dreams that have hardly died in the interesting 19 centuries. Despite the meddling of a nervous and presumptuous American producer, "I, Claudius," remains a rich and respectable sensation.