I'd like to hear a lawyer's opinion - or a legal explanation from Royal Mail.
The fact that a stamp has previously been peeled off a backing does not necessarily mean that it is now being re-used fraudulently. As well as the cases already quoted, I have in front of me an unused 1st-class stamp stuck to an envelope over which some coffee was spilled before it was posted. That envelope was scrapped, except for the part with the stamp on, which was clean. So the valid, unused stamp is there for me to steam off and use, even if the perforated panels come out.
If I did so, and the recipient, having taken the letter from the postman, then refused to pay the excess, the Royal Mail would have no case in court because they could not prove fraud.
I think this is a bluff.