First, contact whichever department of your base handles movement of personal property. They can let you know about storing your Spyder while in Korea and then the rules about shipment. I don't know the exact Status of Forces agreement specifics with the UK but you may not have to do any mods to ship and ride there. My internet connection is extremely slow right now, but the website that can give you info on BRP dealers is
http://uk.brp.com/spyder/. Even if one is not really close to you there is a very good possibility that there will be people on your base who do their own maintenance. It isn't that hard to pick up a digital copy of the service manual which should cover you for basic oil changes, etc and then ride to a dealer if you need something beyond that. After all, even in the states we sometimes have to travel for dealer support.
Of course you could always sell, but you are going to be in an area with some awesome scenery that just begs to be seen on the back of a bike. And consider what a sale would cost you, are prices such that you would take a loss? Lots of reasons to ship it and enjoy.
Thanks for your service, best of luck to you in whatever decision you make.