It is strange that a person would receive a fine for receiving a letter with a "counterfeit" stamp. That would make for a very nasty revenge operation - send your worst enemy letters with counterfeit stamps to cause them to incur a five pound fine.
put your enemies' address as sender (not legal advice)
"Pay us $ (even though legally you don't have to), otherwise we'll be keeping and/or destroying your stuff".
Extortion :)
Not to mention that this way targets the person that is far more likely to pay, and far less likely to challenge a false counterfeit charge.