adslguide is another good resource. I've heard good things about Nildram - although as I'm on NTL it's a second hand recommend!
---------- Post added at 16:44 ---------- Previous post was at 16:41 ----------
Have you checked adsl availability on BT's website yet (link on adslguide)? I plugged in my postcode and it said I'm unlikely to even get 512K as I'm miles from the exchange. Worth checking.