LONDON.- March is clearly the month for artist Joe Tilson. To begin, there is a major show of his work entitled Tilson: Pop to Present at the Royal Academy, on view until April 12. But then Tilson, while he could hardly be described as controversial, is a bit of a problem these days. Having had his period of enormous popularity in the Sixties, when he was a shining star of Pop Art and represented Britain at the 1964 Venice Biennale, he suffered from a corresponding trough of neglect in the years between. In fact he has been continuing along the lines that he laid down then, working in simple patterns and bright colors, and has found constant patronage as an adjunct to smart modern decorators all over the world. Indeed, in recent years he has been better known abroad than at home. Significantly, this is his first retrospective in Britain. The present show consists of some 40 works, covering his whole career from the key Pop works of the early Sixties to the most recent paintings on canvas. Meanwhile, this main event has spawned a number of lesser, commercial ventures. At the Beaux Arts Gallery Tilson is showing more of the same, under the general title of Conjunctions, on display until April 20, and at the Alan Cristea Gallery there is another sort of mini-retrospective, confined to works evoking Greek and Italian imagery in prints made between 1989 and 2002, on view until April 13.