In my opinion EVERY air gunner needs one if you have to travel to shoot - unless you know the gun is underpowered. It would be extremely rare to get stopped by plod - but in the event you get your gun checked it could well be sold, oops sorry destroyed for the slightest amount over the 12ftlb threshold.
It's best to consider it as an essential accessory that can give you peace of mind. You may well find that your gun is very underpowered - and choose to do something about it - or likewise overpowered and do something about it. Please be advised that if you ask a registered firearms dealer to check it for you, he is (allegedly) required to steal-sorry I mean confiscate the gun - but what happens to it then?
All for the sake of Â£40. you could also buy one, check your gun - and sell it again.
EDIT: I'm cynical early in the morning