I am pretty certain they do.
I am incredibly annoyed with BT and I will probably be migrating because of it. I don't know exactly what times they cap, but I noticed after 6pm I get about 50kbps (I am connected at the full 8MB).
Right now I am getting between 2kbps and 50kbps, which is unacceptable, seeing as I definitely do not go over the download limit.