If you like your Sibelius you'll love this guy's music. I just thought I'd get that out of the way first. I'm absolutely sure Sibelius made a massive impact on this composer.
Edmund Rubbra is probably the most unjustly neglected British composer who ever lived. No, not Gray. Not even Bowen, though he comes close. And to think that such an obviously great composer was quite popular with the public during his lifetime has slipped to being so obscure is really a shame. He was very prolific: 11 symphonies, several concerti, much chamber music, a lot of vocal music... the symphonies are probably the keystone of his output. What we see developing is, like Sibelius, a growing severity of expression, though none the more harmed for that. Indeed, what replaces the open emotionalism and radicalism of the first two symphonies is replaced by great sublimity and organic development highly reminiscent of late Sibelius, but still distinctly his own.
For me, the starting point was with his 6th symphony, whose second movement is very poetic and sublime, not to mention the other movements (four total). His last symphony, the 11th, is rather difficult to really get into; its one-movement, 15-minute form is most intriguing and mysterious to me. The 9th symphony is rather like Mahler's 8th symphony, I think, just at first listening; singing practically the whole way through, and much "narration" with musical accompaniment for a bunch of the time. I think it relies ever so slightly too much on the text ("Sinfonia sacra," as it's called, takes its text from the New Testament Gospel, I believe--not sure where from, though). There are some great moments for the music, though, and it makes the symphony worth listening to, in my opinion. He just wrote better stuff.
Another piece to really look for is the viola concerto (and of course I of all people would bring this up). In my humble opinion it is far greater a concerto than Walton's, and maybe even Bartok's. That was my big introduction to Rubbra in general, and it was a good one (the disc I listened to it on had it paired with Walton's viola concerto as well as the first recording of Rubbra's Meditations on a Byzantine Hymn for viola solo; Lawrence Power was the violist).