Do your research!
The sooner all these idiots stop signing up for the top 5 broadband providers the better, based purely on either because they see nice glossy ads in the papers or on the telly and think it's safer to go with a big company or go with whoever is the cheapest. Contrary to what some people have said here there ARE still a few good ISPs left - most of the Enta resellers, IDNet, Zen, AAISP, fast.co.uk, maybe Newnet and Be (if you can get it). Take your time and do some research on sites like dslzoneuk, thinkbroadband etc. Read and post in the forums, get word of mouth. Get a one month contract so that if you are not happy, you can migrate.