I'm surprised that you have apparantly 'gone up to the wire' and let your existing subscribtion expire before exploring the possible alternatives.
I've not had personal experience as I'm still on dial up connection! However, my friend has a Telewest contract that also includes satellite TV so one question is does your expired contract just provide Broadband or is there anything else?
Another point is does your existing connection include a free email facility that you use frequently? If so and you decide to change, you'll have to advise your new address to all existing contacts or arrange for emails sent to existing address to be automatically forwarded to new address if that is possible.
Apart from that, I don't know of any other snags about changing your provider, although there may be other members who have had bad experiences!