Yes, there are lots of broadband ISP`s, which as you say, use the BT exchange and line. However, this does not mean that you should use BT broadband. Other ISP`s offer excellent service, and at lower cost than BT.
I recommend Pipex; why?, I have been with them now for about 11 months, and find them very good. They have an excellent promotional offer at the moment that you should consider.
For only £23.44, you get you BT line activated, plus, a free ADSL external modem, and 2 microfilters. Their subsequent subscription price is £23.44/month; which is less than BT. These prices include VAT.
As I recall, the initial contract is for 12 months, but you can buy yourself out of it for about £50 or so.