I've always been of the opinion that for any given law to be effective you need at least one of the following to be true:
1. Very high chances of being caught, or
2. Very strong punishments if caught.
Providing you have 1 or 2 (or both), many people will be put off from doing it. The £5,000 threat will deter many, which presumably is the desired effect.
I'd also say that "the law is an ass" is, as a line, overplayed. I've know a few magistrates over the years and most of them would have no hesitation in saying (in terms) "you're taking the piss mate, here's a £5,000 fine".