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.
"Pay us $ (even though legally you don't have to), otherwise we'll be keeping and/or destroying your stuff".
Extortion :)
put your enemies' address as sender (not legal advice)
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.