Another vote for Be, I've been with them since 2006 and they've been great - the only hitch is that they don't seem to be able to run stable DNS servers, so I use the O2 ones (which is the same network, bizarrely).
Be staff have stated on many occasions that they have no download limit and that their "Fair Usage Policy" is only there if somebody is downloading illegal content - but even then, Be won't pass on your info without a court order.
Great ISP, and I'd go with them every time, even if I didn't get the full 24mb that I do currently.
If you're outside their limited coverage area you can only get the O2 Home Broadband Access product though. It runs at a maximum of 8mbit/sec and is £22.50 per month (£17.50 if you are an O2 mobile customer).
They had some SERIOUS slowdown issues with this earlier in the year. I'm reluctant to recommend them but service before and since this time has been excellent.