7Digital is one of the sites that you can buy music from online, and offers a mixture of DRM free MP3 files and WMA format files with prices starting at 50p per track.
What is of interest is that according to PC Pro, 7Digital has entered into discussions with broadband providers to offer downloadable music to customers as a part of their broadband deal. It looks like this would mainly be a billing arrangement, so that the tracks you bought on 7Digital would be added to the monthly bill from your provider.
Partnering with the broadband providers could be a clever marketing move, since if they can be mentioned as a simple source of legal music downloads on any file sharing letter sent out, the sales at the company mentioned are likely to increase.
Semi-tongue in cheek but also possibly serious - the ISP could even offer unlimited at a fixed price. If things got out of hand they could merely apply their standard FUP to the customer's usage as a whole!
Non-FUP ISPs would need to have a variable charge presumably, but even that could be banded fixed cost like bandwidth itself.