I switched to iDNet having got fed up with another provider that advertised the fastest speeds in the country, but regularly delivered an appalling experience due to oversubscribing the area.
Although iDNet are more expensive than most they promised no oversubscribing and UK based customer support. They have been true to their word, delivering a very consistent connection and providing very helpful customer support on the rare occasions where there have been issues. In my experience they do actually ring you back when they say they will!

I highly recommend them.