Having used them for years - I would recommend IDNet - they managed to get my ADSL functioning quickly and efficiently, after BT had taken 6 weeks to not install it... They do what they say they'll do, if you've got a problem, you ring up and speak to someone who's name you know, who tell you what to do when things screw up.
They don't do anything amazing they just do the really simple stuff, really well. If there's a problem they sort it.
They're not the cheapest, but neither are they horrendously expensive. And I'll bet they're at least as fast as any other company using the same BT lines...
Ranty recommendation over - they are officaly quite good...
I'm with BT Internet and I wouldn't recommend them, I just haven't got annoyed enough to move yet.
The best reputations for a professional service, rather than the usual are Zen and Andrews and Arnold. Neither of them are particularly cheap but you should get what you pay for. I don't know about contention ratios.
When you say at least 8MB, I think most ISPs offer an upto 8Mb service, but what you get may be considerably less. I think that the higher speeds (upto 20Mb) are only available in a few areas at the moment.
Be Internet and their "white label" operations, such as O2, seem to be well regarded.
Type you phone number into SamKnows to find out who offers service in your area. If you want a real "at least" 8Mb connection on a BT line (not an up-to 8Mb service) then I hope you are within about a mile of the exchange and you are looking at an ADSL2+ service from Be or O2 (same people), or maybe C&W/Bulldog if you can persuade them to sell it to you.
That SamKnows link is great. I am one of the lucky few has had a great run with Sky for many years and two house moves. If you have a Sky Subscription already and they do LLU on your exchange, then they really are good value for money. However Customer Service can be bad at times, although I haven't found the need to use it more than a couple of times in 6 years.