QUOTE(Swell Box @ May 14 2012, 09:33 PM)
I think you'll find that all of these services are provided by BT, which they sell wholesale to other 'providers'. So you'll be using BT whoever you pay the bill to. (This is why BT vans have now been rebranded 'Openreach' as they service all providers' customers.)
I an fairly IT savvy, but on the odd occasions that I have had problems BT have been very prompt and helpful. I cannot speak for other providers.
To clarify, although the line still belongs to BT Wholesale, you don't have to pay anything to BT Retail if you switch provider. (Openreach is the part of BT Wholesale which deals with the last-mile connections from exchanges to properties.)
Some companies (Sky being one) may take over the line entirely. I discovered this on moving into one house, that I had to pay a full connection fee to get it transferred back to BT Wholesale. Clever move by Sky, creating an incentive for the new occupier to just buy their package instead...