you'll obviously need to have your broadband service via the BT phone line - but you can choose any broadband service provider you like. All of them must buy the broadband access from BT wholesale anyway.
Take a look at what's on offer - and don't discount BT themselves, you can buy a broadband service directly from them. One thing to be aware of is the current advertising for 'broadband' at only £15.99 a month. Whilst it's true to say that the service is an ADSL one it's not as fast (at 128.0 kbps) as the 512.0 kbps service that we have all got used to calling broadband.