Some years ago, the standard security check of tourists at Buckingham Palace detected a revolver and live rounds carried by an American man. It seems that he was a US policeman, and had brought the items over on a flight with no problems: it seems that he had just shown his police ID to the security in the US airport who let him through with no checks, and, apparently, he had no idea that guns and ammo were illegal in the UK. He was most offended when they were confiscated.