You may also want to add Lennox Berkeley and William Walton, as fairly major composers who wrote mostly not for the guitar, but who did, of course, give we guitarists some nice little gems, "masterpieces" in fact!
And, on the subject of British composers, although we have yet to hear his new guitar piece, Harrison Birtwistle wrote one (and he is also a major composer!).
Edit! I forgot, Birtwistle also wrote an earlier guitar piece (for his son), just a short one, called "Guitar and White Hand."