I don't know of anything. I'm happy with pipex. A friend seems equally happy with talk talk. But a word of warning about any change - will you still be a BT customer for your phone line? If NOT, then you will lose some features that you might not want to lose - such as the free answer-phone service on 1571.
Mention it only because another friend switched to a different provider to get cheaper calls, but hadn't realised he would lose BT features that he used. He's now switched back to BT.