Los Angeles celebrates Britain's contributions to arts. The British are coming for a 12-week arts festival in Los Angeles, with premi`eres galore and guests ranging from Prince Edward to artist David Hockney, conductor Simon Rattle, and comedian John Cleese.

The newspapers are filled with reminders that Britain gave Americans the Beatles, the Rolling Stones, the Kinks, and - oh - their language. The columns ask what Charlie Chaplin, Cary Grant, Boris Karloff, David Niven, Richard Burton, Charles Laughton, Michael Caine, Michael York, Alfred Hitchcock, and many others have in common. Answer: They were all Britons who settled in Los Angeles.

And the stat sheets show a record number of British expatriates (350,000) here, carrying the torch of cross-cultural collaboration in a variety of arts.

So why not sound the trumpets, arrange a royal visit from Prince Andrew and Princess Sarah, and celebrate Britain's artistic achievements with 12 weeks of music, opera, theater, arts, crafts, design, film, video, TV and radio? Whew!

Is it all just an excuse for yet another international festival on the heels of last fall's 12-week Los Angeles Arts Festival, itself spawned by the 1984 Olympic Arts Festival? Or something more?

``It just seemed that the time was right to nurture our relationship with one another,'' said Donald Ballentyne, the British consul general here, who decided two years ago to coordinate this UK/LA - Celebration of the British Arts. ``We went through a very bad period economically, with the reputation of ... always being on strike. But all those days are past now,'' he adds, asserting that Britain's economy is now the fastest-growing in Europe. ``The object is to have America look over its shoulder from the Pacific Rim and think back on Britain as still an economic and cultural force to be reckoned with.''

Thus the UK/LA Festival is being launched today with a retrospective exhibition of the work of David Hockney at the Los Angeles County Museum of Art. The festival will conclude April 30, following the final West Coast premi`ere performance of the Academy of St. Martin in the Fields Orchestra, conducted by Sir Neville Marriner, at the Ambassador Auditorium.

In between, 55 Los Angeles companies and 29 British companies will be bringing audiences five world premi`eres and 20 American premi`eres.

Highlights of imported performances include ``The Mikado,'' directed by Jonathan Miller (March 10-13 and 16-20); the City of Birmingham Symphony Orchestra, conducted by Simon Rattle; whimsical sculpture by Boyd Webb (March 22-June 19); Trevor Pinnock's English Concert; avant-garde dance from the Irie! company, a black troupe; monologuist David Cale; the Welsh Pop group David Loves Jezebel; and comedian John Cleese discussing the healing effect of humor with American author Norman Cousins.

Among the highlights of local offerings are Los Angeles Philharmonic performances, under director Andr'e Previn (principal guest conductor of the Royal Philharmonic in London), of works by Britten and Walton; the Los Angeles Chamber Orchestra (whose director, Iona Brown, is also music director of the Academy of St. Martin in the Fields) performing Elgar, Purcell, and Vaughan Williams; and the Mark Taper Forum's American premi`ere of ``Made in Bangkok,'' winner of the London Theatre Critics award as ``best new play'' in 1986.

``What's unusual about the festival is that it's as much about local groups doing something of English origin as them bringing over companies,'' says Gordon Davidson, artistic director of the Taper. ``So the festival stimulates our own work ... and gives it a bit more focus and attracts a wider audience.''

``Some of our British visitors are going to be amazed at what's going on here, and they're going to take some things away with them, too,'' says Ernest Fleischman, artistic director of the Los Angeles Philharmonic.

The idea for the event was that of Consul General Ballentyne, who had begun thinking of British contributions for last fall's arts festival here and got so much positive response that he decided Britain should have a festival of its own. Since David Hockney and the Birmingham Symphony Orchestra were already scheduled, they became two of the pillars.

And though the festival's intentions are primarily artistic, there is a backdrop of commercial underpinning. Reporters at two pre-festival fetes were treated to tote bags full of British goodies - mango and lime chutney, whole-wheat crackers, books of English recipes, and guides to British restaurants in the area. A local department store will be promoting British merchandise during the festival, and some grocery stores will too.

Tickets range from free to $50.