As I understand it, Plusnet are a subsidiary of BT. They are based in Sheffield and have a UK call centre. I know of two people who use them and they have both been delighted with customer service whenever they have called them. Hope that helps.
I have been looking at perhaps changing to Plusnet, but it might be wise to check any small print, especially on special offers. Apparently there is a part regarding 'local' area rates, which might make a difference to the final contract, depending on where your location is in the country. Other than that, there seems to be some decent reviews on Plusnet.