Simple to prove, when they confiscate the rifle and ammunition that you have with it as part of a wider investigation. Measuring velocity is very simple and any Police firearms laboratory could do it.
You are very unlikely to be using vastly different ammo when out Stalking so the ammo they seize is what you are using.
Can be bothered - probably not. However if they are looking at charges why not add another one or two.
It’s perfectly possible to shoot red deer with a 223, 22-250 or a 22Hornet. Plenty where shot in the old days, expect plenty still are. But is it legal - no.
Air rifles. Easy enough to tweek to add quite a few fps so they are well over 12 ft lbs. Is it worth doing and risking breaching firearms act?