What ever you decide to do, do NOT go with ADSL Max. Highly unstable technology and very unreliable. I'm on ADSL Max with BT and I have disconnects at least 3 times a day. That's guaranteed. The best choice at the moment most definitely is what Rambo just mentioned, Virgin Media. ADSL providers are decent, however ADSL is going out of fashion now due to the introduction of ADSL Max. Nearly all DSL providers are replacing their packages with ADSL Max now which is a HUGE mistake on their part. Once my contract is up with BT I too plan to move over to Virgin Media.
Zen were, from what I've read and heard by word of mouth, the current best DSL provider, however from what I hear these days they've really gone down hill now with their ADSL Max packages too. They introduced that technology to DSL providers way too soon in my opinion. Cable seems the stable future for Internet access in the UK for the time being.
Breaks.