My parents have Talk Talk. It always seemed pretty decent to me, though that was probably relative. Before TT bought them out, it was a Tiscali connection. Now THAT was bad - not only was it sluggish, it would cut out for a few minutes at a time multiple times a day. The TT connection is actually reliable. While it for example cannot stream a video as quickly as it plays it (forcing a few minutes of waiting with it paused while it buffers), I don't think it's 20kbps slow.
However, in Scotland, me and my flatmates are with BE. Absolutely incredible connection. Very reliable, very fast. Their main issue is their coverage - if you're in a rural area, such as my parents' house, BE is more than likely unavailable. S'one of the benefits of living in the centre of a city, though.