Virgin Media. At the time I switched I needed more bandwidth for work - dealing with multi-gigabyte blobs all day; I was with BT, but BT wouldn't let me upgrade to a gigabit fibre connection, and the City Fibre network which is now everywhere wasn't yet in my street.
Many ISP will also auto-redirect un-allocated domain names to their own websites. Others will ban most inbound connections with a port under 1000 to prevent self-hosting/video-surveillance users.
Annoying if you are trying to bring up a remote domain server, and thinking WTF while checking things out in dig. lol =)