Yes the floor in your plan is large enough to drive a coach and horses through. Now the police on this side of the pond are neurotic about persons with firearms be they old new or fake at an Airport or any where else for that matter and this is understandable. For openers firearms and us Brits are a thing for the heavily regulated sporting fraternity, they are not very commonplace for the average law abiding Brit. So if you do turn up at the Airport with a gun without the correct documentation for example a letter from your God if you have one, failing that our Queen and the prince of Wales and every member of the Brit parliament of course countersigned by your grandfather's grandfather you may be in for a bout of unrestrained nastiness. Now I know what an American accent is but to security operatives you are a dubious Foreigner with a gun needing to be neutralised. To this end you would more than likely be taken in hand by about four rather large police officers who will drag you to the ground making exerted attempts to put large cracks in your skull with the floor because you may be about to do something very un British like.
Buy a gun by all means but go through the correct channels to take it home.
