A good start is click here
and click here to compare services and prices.
A few days ago Radio 4 run a feature about ISP's in which it was pointed out that the devil lies in the small print. That is to say, some of that the helpline of ISP's tech support are premium rated. If something does go wrong, that's where you find yourself paying exorbitant money, despite having chosen an attractive package.
For what its worth; I am on BE (switched from Pipex) pay 14.-- and have a download speed of more than 11 GB. Happy with it.