AFAIk, recodring a telephone cal without the permission of the people involved is also against the law (which is why businesses, BT etc always inform you that this call may be recorded). At the very least it will be inadmissable as evidence in a court case, and an extreme case could see the police using their powers under the prevention of terrorism act against you for recording a restricted conversation.

IMHO, let it go. We mere mortals can never win against the almighty plod, so no point giving them a bigger stick to beat you with.