Britain's teletext: a billboard, a news service, a mail order outlet. . .

''You ain't seen nothin' yet,'' said Al Jolson early in the first talking movie, ''The Jazz Singer.'' He could have been talking about the British invention teletext.

Already shops in London advertise on commercial television's Oracle service on Channels 3 and 4, then leave TV sets in their shop windows permanently tuned to the ad.

It's another way of trying to draw customers inside.

BBC's Ceefax is experimenting with broadcasting programs for microcomputers. Plug the computer into the TV set, and you can program the show into the computer. Simple.

But Humphrey Metzgen, in charge of selling advertisements for Oracle, has more visionary ideas still. In a long interview he ticked off the ways he wants to harness the new idea of teletext:

* Instead of mailing out letters to 20,000 housewife agents at $1.50 a time, mail order houses could update catalogs by telling the women to dial up a certain ''page'' on Oracle for the information.

* Each region in Britain could have its own bank of classified advertisements - shops, restaurants, etc. - on Oracle pages.

* Credit card companies could save (STR)20 million ($32 million) a year in stolen cards by installing Oracle sets in stores. Cashiers could instantly check a card against a screenful of stolen numbers; or sales assistants could run cards past a handset that would ''read'' them and compare them instantly with stolen numbers on file.

* A single Oracle operator could beam advertisements to roadside billboards in any location. Each billboard would have its own small antenna and decoder.

Advertisers could order a certain kind of message beamed to billboards in morning rush hours, a different one at midday, and a third one at night, each aimed at different audiences.

Today, Oracle fills between 30 and 40 ''pages'' of advertising each day. It still loses money to the tune of (STR)1 million ($1.6 million) a year, but Mr. Metzgen hopes to break even in 1985. Recent innovations include broadcasting British Airways arrival times from around the world, based on pilot telexes from aircraft in flight.

On election night, both Oracle and the BBC Ceefax system will keep updating election returns through the night.

Teletext is at its best when it has time to prepare, and handles a large volume of changing material. Graham Clayton, editor of Ceefax, has had his own reporter at major sporting events this year, and will have them at Wimbledon this year as well. Viewers with teletext will be able to summon up the latest Wimbledon scores day and night.

And the Philips Company is now making (and will start selling in August) a set for (STR)699 ($1,150) which prints out the contents of any screenful of data.

So the question arises: Will teletext replace newspapers?

Graham Clayton does not think it will eliminate the serious, analytical, in-depth paper. ''We are a headline service,'' he said in an interview. ''We cannot get into depth in our short reports, though we do put longer pieces of background on Channel 2.

''The printout set will be good for recipes, for instance, so that cooks can go off to the kitchen to work with them.

''But replace the daily paper, which can go into more depth? I don't think so.''

Peter Hall, editor of the Oracle news service, sees teletext as providing an updating service for the serious newspaper reader.

''We fit well together,'' he said. ''We complement the papers. We don't replace them. I aim for an Oracle service which is not the Times or the Telegraph, but more like a mix of the Express and the Mirror. . . .''

One step forward will be when a ''browse'' button is developed.

At the moment, it is not possible to move from one ''page'' to another without dialing up through the handset. A ''browse button'' would allow viewers to move through the pages, as though flipping the pages of a newspaper or magazine.