Anybody including BT. They have to go through the same BT Openreach crap that Virgin, O2 et al deal with.

Everybody seems to forget that BT is not one company - BT Openreach and BT Retail have as much to do with each other as Railtrack and Virgin.

However, if your exchange is congested then you can get better speeds by simply paying more. Ask your provider for a premium product with a lower contention. Most people won't pay, so those with the deepest pockets get priority.