I'd like to second Spineur's recommendation that you explore the music of Frank Bridge. He was Benjamin Britten's most important teacher and mentor, as well as being a very fine composer in his own right. Elgar and Vaughan Williams are people you should explore. One composer I didn't see mentioned here is Karol Szymanowski, another is Leos Janacek. And although he is a little early for your timeline, I would suggest you might want to investigate the chamber music of Max Bruch. I am not very excited about his much more famous concerti and symphonies, but his chamber music is truly amazing. In fact, I find that I am constantly impressed by the lesser known works of already recognizably great composers--Schubert's Masses & Dvorak's Cypresses among them. Also, Wilhelm Stenhammar's six string quartets, and, of course, Camille Saint-Saens, whose last published work (1924, I believe) was an oboe sonata. People think he died much sooner than he actually did!
