Most of the High Street banks now offer 'basic' bank accounts. The Financial Services Authority publishes a booklet which explains them and tells you which banks offer them. You can download it here:
http://www.moneymadeclear.fsa.gov.uk/pdfs/bank _accounts.pdf
Even undischarged bankrupts can open a basic bank account with a few of the banks listed, so you shouldn't have much of a problem simply because of a poor credit rating.
I've got a basic account with Barclays. It was a quick, hassle-free process to open it. Check the table, in the FSA booklet, to see if it would be right for you.
Barclays is one of the banks which allows undischarged bankrupts to open an account. The other two are Cooperative and Nationwide. I suggest trying those three banks first because they're the ones which seem least likely to be concerned about your credit rating.
Chris