THe Fender Hot Rod Deluxe and Blues Deluxe combos are well thought of as very loud, reasonably priced clean amps, to run pedals into for overdrive. Both come in at under £500.
If I was in the market for a sub-£500 amp right at the moment I would DEFINATELY be looking into the new Bugera range of amps. I have heard some very, very good reports of them, and they are ridiculously low priced. The pick of the bunch seems to be the 333, which offers 3 channels of 120W tube amplification. From many accounts, they sound excellent too, and seem to be built at least as well as the equivalent priced Fender, Laney, Peavey or Marshall offerings.
I have not played any of them yet, but I intend to.