Might not be as simple as that, because dont forget he will have a 12 month broadband contract with BT.
Also i think you can only use one service provider with broadband, because i have 512k broadband at home and at work, and i cant login at home using my work setting and vice versa. So i think you would need to cancel the contract with BT before you could try another ISP. And if you are in teh 12 months they will charge very nicely!
I might be wrong but this is the way i understand it.