I doubt you would need to be licensed as you are a member of the defence force and at least over here in Aus,
carriage of what would normally be prohibited firearms is allowed.
How would you issue a license to him in the UK when pistols are illegal
so no such license exists ?
Easier to do it under the legislation covering the Armed forces.
After all, the SAS and others carry what are normally prohibited firearms
when in civilian clothes - and use them !