If cable is an option for you, then I would opt for Virgin Cable broadband. I don't rate their customer service at all, but the connection is very reliable and very fast (you get the speed you pay for). My service over a year has been so reliable that I've not had to call them at all.
I pay for 10megs and I get - on average - around 9.6 - 9.8 megs. That's something that an ADSL phone-line based service can never get you (the rated speed). There's also room for expansion, so if I want to upgrade to 20mb or 50mb I can do at any time.
I have a phone with BT but they will only offer me 1mb where I live even though it's in a town centre, so I pay £20 per month for my service, but the value is better if you bundle the service with a phone or TV, obviously.