I had recently visited Bisley, UK to attend an F Class event. I travelled with Emirates and they are very efficient as far as clearance and Documentation is concerned. I used their online declaration form and uploaded permissions from both ends, along with details of my Fire Arms Licence and invitation letter. After 3 days I was emailed an authorization letter along with a copy of my Declaration form with instructions on how to go about it at the Emirates Desk at the Airport. Upon landing into the UK, the Fire Arm and Ammo box was at the Customs, I showed all necessary paper work and within 10 mins i was out of the Airport.No Hassle at all.
Upon my return flight, at the check in desk, I again showed all paper work, then went to customs, where I also got my VAT refunded for some Ammo I had purchased. After inspection of all documentation and the Fire Arm itself, an Emirates staff escorted me to the scanning machine where the Fire Arm and Ammo box went through the scanner. At that point me and my Fire Arm departed. It all went smoothly and this was at London, Gatwick.
A word of advice: Those into hand loading often carry either bullet tips or empty brass into their regular checked in luggage. International flights usually don't have an issue with this and you don't normally need any permission to carry something like that. A couple years back i got into trouble with just plain tips and brass during a transit my flight had made! It was a long ordeal, at the end I got away with it however, my tips and brass were confiscated as it was considered "Ammunition"! So double check with the airline before you carry something like that if you don't have prior permission.