The term "fine" seems confusing here. If you look at the form sticker, it seems apparent that £5 is just the on-delivery postage rate for an unpaid item. Different postal services handle this differently, but it's not unusual that a mailpiece that's lacking sufficient postage will be delivered to the recipient if
they pay the postage, and it's also not unusual to make that a higher rate to account for the extra work involved in notifying the recipient and collecting it. Under some postal systems you can do this totally intentionally by marking a mailpiece COD.
Maybe Royal Mail really does call this a fine, but it seems like it's just a typical higher "postage due" rate, thus the still rather nominal amount. Paying is optional for the recipient, they could just ignore the notification and it won't be delivered.
The use of this approach for counterfeit seems sort of unwise considering the accusation involved in counterfeit postage, the USPS returns the piece to the sender in that case. But the items on the sticker make me think that Royal Mail has a general bent towards offering delivery no matter what. USPS would also return to sender if there's no postage at all, assuming the piece doesn't indicate the postage should be paid by the recipient. But you can see that the £5 sticker here is the same one used in that case.
Sometimes the situation is complicated by postal policy, for example UPU policy for international mail tends to strongly prefer attempting to deliver a mailpiece over returning it, so "postage due" stickers seem more common on international mail (particularly since the international rates can be confusing and it's easier to accidentally underpay).