"Rock Follies," a five-part music and drama series about the vertical and horizontal adventures of a three-person female rock group, makes for basically brisk and cheerful diversion and is attractively produced.

But the series is more interesting for what it represents than for what it is. After inundating us with the creme de la creme of British television for years, the Public Broadcasting Service (PBS) is now providing us with programming that serves to indicate the level of ordinariness sustained by British TV.

In other words, we are seeing the upper edge of their mediocrity and, as one might suspect, their mediocrity is better than our mediocrity.

"Rock Follies," which airs in five episodes this week at 11 o'clock nightly on Channel 26 (tonight's premiere is actually about 75 minutes), is a bright, lithe and lightweight entertainment that takes three gal pals who might have been characters in a '30s musical about making it in the theater and turns them into three gal pals trying to make it in the indubitably grubbier world of '70s rock.

Though hardly a feminist manifesto, the show, written by American-born Howard Schuman, provides us with three essentially strong and assertive women capable of all kinds of initiatives, including sexual. The show's language is zestier and its dramatic resolutions less pat than in the average American series, mini or maxi.

"The Little Ladies," as the trio ironically dub themselves, are nimbly portrayed by Julie Covington, Rula Lenska and Charlotte Cornwell. They meet at rehearsals for a seedy Broadway revival that hits a well-deserved iceberg, later become a team which finds deft ways of dealing with rowdy crowds, snoopy journalists, and with the decidedly luggish men with whom each has become encumbered.

"Rock Follies" has been scheduled as part of the 15-day PBS fund-raising ordeal, "Festival '77." a collection of acquisitions and imports. Like most British shows aired by PBS stations, whether quality stuff or just competent fluff, "Follies" is both a pleasure and a sign of scandal. It's enjoyable in its own right, but it serves to emphasize again the failure of American public television to generate and showcase comparable demestic creativity.

Sooner or later they're going to reach the bottom of the British barrel, "Rock Follies" being clearly from mid-range. Heaven only knows what we'll see on PBS then - perhaps a Polish variety hour, or maybe a little Ibsen a la Iceland. How many languages does Alistair Cooke speak, anyway?