Sudden said:
http://www.adslguide.org.uk/ is a good site for this stuff. plenty of comparison's and a message board dedicated to finding the right ISP.
another vote for that website. Top notch stuff. I personally have used BT, Tiscali, Nildram and freedom 2 surf (f2s).
I have to say Nildram have THE BEST customer support bar none. I dont care what any surveys say. Nildram is top notch.. always call back asap, and do curtosey calls a week after the fault has been fixed. (the fault i had was a BT one and they chased it up all the time). There performance is always top too.
Freedom 2 surf, have top performance however their customer support is good when you can get through. Afternoons is 15-40 min queues. and they shut at 4:30 or 5:30 some days. Which imo is totally unacceptable. other than that its good.
Tiscali throttle the speed A LOT. At my parents house where i used to live, i could get download speeds as slow as 3kb/sec - WORSE than dial up!!! and 200 ping in games. Personally as an avid online gamer i will never used tiscali again. However for cheap unlimited broadband for the occasional user its alright i guess.
BT. Speeds where ok, tech support was ok. BT yahoo tech support was better than BT imo. Probably the same call center but the queues where definately shorter. And When they say they will give you a discount on your next bill, check your statements. They forget.
I am with F2S atm as its provided by my company. However if i was to pay for an ISP. I would go for nildram.