> If this law, all by itself, was perfectly enforced, the UK would bankrupt itself just on the extra spending in the prison system.

That conclusion ignores the interdependence of those effects. If people knew that it's going to be perfectly enforced, then almost nobody would do it anymore.

(Not that I'm a proponent of this particular law; just pointing out a flaw in your argument. Somebody wrong on the internet -- gotta reply!)