Try www.zen.co.uk or as already mentioned www.nildram.co.uk both very good ISP's. You can shop around and get cheaper but most have put into place very low download limits (which defeates the whole point of having a fast connection in the first place IMO), or you find that the contention ratio is very high and throttling systems are in place where you only get quick speeds at fixed times.

I'm afraid the UK is years away from getting good a broaband setup while compaines are still allowed to set broadband activation limits it is not going to get much better .