don't forget that you'll be limited (sometime severely!) by what services are available in your area. Broadband isn't available everywhere yet, and not even all parts of London can get all the options!
That said, I use clara.net and have had no problems with them and can recommend them!
NDO seems to be the best deal for those who only want a 1-month contract... you pay for the modem, but pay no setup fees, £25/mth, and apparently great service and high speed.
telewest very cheap for new customers at the moment... and BT is free to install at the moment, but line rental slightly more than BT... Broadband heavily advertised over here, on radio and tv, at the moment, so best to play it by ear for the latest deals etc
BT broadband? Oh please! Their bandwidth limit: 1 GB per month. Per month! I usually go through more than a gigabyte in a week.
Pretty much all of the big companies here are crap. Just look at the ISP review sites, and they pretty much all get "5/10" scores. Their service tends to be rubbish, and because of the demand for these large companies, you're put on a long waiting list to get connected (often several weeks).
What's the point of being a sheep and joining one of the big players just because they have the cash to bombard us with mass advertising campaigns?
Have a look at ispreview.co.uk and see what your options are before making any purchase... you won't be sorry.