If I may offer some information from the other side of the pond, when the gun comes into the UK under any other heading than personal possessions (which I think needs to be traveling with you as part of your luggage - check this - and if so you will need some kind of UK certification, eg. a visitors firearms permit)it will be liable for import duty and VAT. The UK customs are very tight about this - don't trust to luck!
If it is being imported into the Uk as a sale or gift, this is fair enough but if being sent for repair or for personal use on a sporting vacation it might turn into an expensive trip!
If it is coming in for repair, make sure your importer has an Inward Processing Relief (IPR) authorisation or you will have to try and recover duty and VAT after the event which is extreamily tedious and not always successful. Quote his IPR number, VAT number (if applicable) and Open General Import Licence number on the packaging and paperwork and the gun should go through undelayed. Don't and you could be looking at weeks of delays, I know from experience!
If the gun is coming in for sporting use, make sure you have the certification required as above and I suspect that if not travelling as checked luggage you will need a customs broker to clear it through for you, at some expence.
Good luck!