HMV, Virgin, Tesco, Woolworths, Napster (as long as you don't have an iPod) they all have download sites, plus the (slight iffy according to the BPI) allofmp3.com
(though whether there's anything there that you would give the time of day to is a different question - depends on what sort of stuff you like I suppose).
Don't forget www.bleep.com for legal mp3s (no DRM) of dancey electronic stuff and www.tunetribe.com for a wide selection of new music (some are WMA though).