No mention of Havergal Brian so far. symphonies 1,3,6,8,10 are all masterpieces of sorts.Look out for the Lyrita reissue of nos 6 and 16 in Feb 2008.
Patrick Hadley's "The Trees so High" is one of my favourite works by a British composer (Lyrita and Chandos). It should be much better known. I have seen it (incorrectly) described as "Vaughan Williams with water"...infact it is a much more personal work than anything Vaughan Williams wrote (with the possible exception of Symphony 9). Vaughan Williams's music, for all its great beauty has a strangely impersonal quality to it.
Also, Phillip Sainton's "Nadir" (a masterpiece in 13 minutes), The Island and Moby Dick film score are all favourites as are Lennox Berkeley's First Symphony and double piano concerto. Arthur Benjamin's Symphony is like the VW No 6 (Benjamin was actually an Australian). Cyril Scott is having something of a CD revivival. John Gardner's 1st Symphony on Naxos is also worthwhile as is Edgar Bainton's movingly valedictory Third Symphony (Dutton).