Just to add to bhg's post...
The distance from the Exchange doesn't make as much difference as it used to,
if BT have supplied "fibre to the cabinet" in your area. Check your local green cabinet...if it looks new, have louvers in the side, its where the fiber is terminated. It probably also a huge great big sticker on the front. extolling the virtues of BT Infinity !
Check with BT to see if you can get Infinity, and they will tell you what kind of speed you can expect. Its important to realise that almost everybody who gets broadband from their telephone line, relies on BT to supply that line, no matter who your ISP is.
I can't really comment about cable broadband as it wasn't around when I left BT a few years ago. I changed over to BT Infinity 1 a year ago and now get on average a download speed of between 32 mega pixies and 38 megas pixies.....more than enough for my needs.