Nildram seem to be pretty good in this respect. £26 a month if you don't go with a static IP, 50GB cap a month in peak hours and unlimited at other times. I believe peak hours is defined as 8am until midnight so you get 8 hours of uncapped transfer which is fair enough.
Also, any of the 50GB peak allowance that you don't use gets rolled over to the next month.
I noticed a nice little bonus when I checked my Nildram portal. I signed up with them ages ago and received an activation date of the 6th of July. I'd been allocated a 50GB allowance despite not actually having the service at the time. When it came to the 6th and my ADSL was active, I found that I'd been given another 50GB giving me 100GB of peak time transfer. A nice cockup, but I do most of my downloading overnight and I'll use my NTL connection for that anyway.
Nildram don't appear to do any shaping that I'm aware of. My line is only 3.5mbit (4100 ish sync speed) so I get 410KB/sec maximum. My speeds are always pretty much over 300KB/sec so I can't complain. However, I don't use torrents or any other form of p2p so I can't speak for that but I do use usenet.