Originally Posted by Carnage

Thank you Ryan. Sadly I was posting from my phone and my overly long reply was cut short.

Indeed I was referring to people licensed to carry/own firearms in the US and their on ward travel to the UK.

You would require a British Visitor's Firearms Permit. And you will not obtain one for a handgun, or any type of firearm if you reason for applying is personal security or self defence. Also handguns are generally prohibited from import.

The only exemptions are for certain qualified law enforcement officers (like air marshals) who require no permit, but surrender their firearm before they leave the airport (there is another exemption for close protection officers for various heads of state but that is not the question).