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.
From the article, sounds like the recipient can pay the fine and get the mail, or not pay the fine and not get the mail. Maybe it'll get returned to sender, for insufficient postage? Maybe it'll be destroyed?
It's unlikely that it will be returned to sender. In the UK it is uncommon to write the senders address on the mail piece, and even if a sender address is procided, how do you know that the sender information is accurate?
The problem is that there is no way of knowing what the letter is. All you get is a slip through your door saying an item of mail is waiting for you and has a £5 fee to pay. If you're like me, you pay the fee, fearful that it might be something important.
You pay the fine if want the post. You don't legally have to pay. Presumably it works like this because they can't always identify who sent the letter.