Depends what you are going to be doing. If you want to download a lot (torrents and any other peer-to-peer traffic) then you want to be looking at an ISP that doesn't use traffic shaping, otherwise your experience will differ vastly from the blazing speed the ISP will proclaim in all it's marketing. That rules out the big ones.
Traffic shaping is a mechanism all the big ISPs use to "Guarantee a fair user experience" for all users. Say they advertise 20MB download speed. That will be 20MB when you are surfing general web pages with text and images and pulling back common content like Microsoft patches.
When you want to download [insert legal content here, guv, not of that Frankie-Vaughn business], then the traffic shaping mechanism will be able to identify this traffic by the shape of it's data over the interweb and the speed of this kind of transfer will be throttled right back to much, much less than 20MB
This is supposedly to stop the filesharers hogging the bandwidth that the ISPs have leased through BT's exchange hardware, and allow the ISP to provide a "Consistent experience" to all it's users, whether they are downloading Gigs of shunties or buying a My Little Pony off ebay.
My understanding (and please correct me if I'm wrong) is that, in very simple terms, BT ultimately bill the ISPs for allowing them through the exchange which BT put in at great cost. I believe this is somehow calculated on traffic, so it's in the ISP's interests to not allow massive amounts of traffic through kit that isn't theirs to keep the overheads down.
Small ISPs don't traffic shape and use this as a selling point, because they are not expecting to be inundated by average Pony surfers because they charge more per month to high-volume downloaders who are prepared to pay the extra for unmetered service.
If you are just browsing forums and ebaying then there's not much difference - go with a telly/phone/surf package from one of the corporates.
Here's a good site which explains it in as much detail as you'd ever need and rates the current UK ISPs.