No need to prove anything to a twat.
So you cant prove it and just running off your gob 
And it’s not legal to carry one in the UK.
The simple answer is yes, you are allowed to carry an air gun in public, as long as you abide by the following 4 rules:
1. YOU MUST HAVE A GENUINE REASON FOR CARRYING AN AIR GUN IN PUBLIC.
If you’re carrying an air gun in public, you must have a legitimate reason for doing so. And this does not include trying to look like your favourite action hero. Genuine reasons for carrying an air gun in public include travelling to or from a gun club, shooting competition or air gun shop.
2. THE AIR GUN MUST BE INSIDE A SECURELY FASTENED CASE OR SLIP.
Your air gun must be placed inside a securely fastened case or slip when transporting. You can take a look at the hard case and slip we sell at The Airgun Centre by visiting our air rifle cases page.
3. THE AIR GUN MUST BE UNCOCKED AND UNLOADED
It is critically important that your air gun is not loaded or cocked when transporting in public. Even a cleaning pellet left in the gun can be classified as “a loaded weapon”, so don’t take this one (or any of these) lightly! Plus, guns that take a multishot magazine will be considered as loaded – even if there’s no pellet in the actual barrel, so make sure you remove the magazine and all pellets from the mag whilst transporting in public.
4. YOU MUST BE OVER THE AGE OF 18
This one might seem obvious, as you’re not legally allowed to own an air gun under the age of 18, but it’s worth making crystal clear that those under the age of 18 shouldn’t be carrying them in public either.
https://www.theairguncentre.com/blog/can-i-carry-an-air-gun-in-public.html
There is a ban on self-contained gas cartridge weapons but these fall outside that or they wouldnt be sold in the UK.