If you cover the address of a franked prepaid-postage envelope and address it to your own destination, do you still need a a postage stamp on it or does the fact that it's prepaid means postage has already been paid?
If by prepaid you mean a business reply envelope then no, it's not valid. The printed address and the business reply licence are linked and are only valid together
The Roayal Mail will probably destroy any item that you attempt to send fraudulently as described.
I used to have to send out prepaid envelopes to parents. They're logged by the Post Office as they're delivered back to the originator.
If an envelope has been franked (as opposed to prepaid), then you could conceivably swap the address between the the franking office and the Post Office, but it probably wouldn't be legal.
I dunno though, i worked for an office once and used to receive lots of mail that people had simply used a business reply envelope but wrote there own address over it.