BT are the only ones I'd go for right now - Bulldog looks interesting but like a lot of new-ish companies their customer service is likely to be the last thing they invest in.
BT are also the only residential provider I can name who have almost 100% up-time - Wanadoo seem to be in the habit of disconnecting the link every 24 hours for a minute or two, and NTL are appallingly badly behaved when it comes to billing and collecting money.
All just limited personal experience though, so take it with a big pinch of salt!
/al