Buy it before you leave the UK. It denies the 'hangers on' and 'camp followers' the opportunity to treat you like you have no choice. To be honest, we found it easier to do over here and you have a fixed point of contact should it all go tits up. I've never liked those slimy ex squaddie/former squaddie dependant types who hang about in sheds outside every NAAFI trying to be your mate. Slimy little ******* that they are. Most large dealers back here are more than happy to deal with tax free cars and don't treat you like shite after you've handed the money over.
You just have to be careful this side. If you buy a tax free car and your posting is cancelled, you have to find the tax mans money (it's happened before). If you buy it once you have taken your posting, even if you are posted back to the UK, you don't pay tax, just have to keep it a year before selling.
My info is pretty old though so you'll need to check out the full regulations.
When you buy in the UK the car is fully equipped for the UK as well as the country you are posted to, mirrors and all other legal things. You also get it registered in the UK with a UK licence plate, so no danger of "Q" plates or problems when you want to sell it later on. I assume you are having a right hand drive car? if so then buy in the UK before you go. I purchased from Natocars many years ago and never had a problem. I also took out premature repatriation insurance in case of a sudden return to the UK, which is what happened and the insurance paid the tax due. if your going to buy a LHD car then your likely best bet is to buy it in Belgium, then buy another tax free RHD car on the continent before returning home which will allow you to get the maximum part exchange price for your LHD car in Belgium.
Buy in the UK before you go, but check the sofa for Belgium, IIRC you are only allowed 1tax free motor per 3 year tour, if you buy now, you are coming back with a three year old car. As long as your present motor is in good nick, wait out and buy in a couple of years. More bang for your buck if you then want to move it on when you are back in the UK. The hive should have all the correct gen.
Orgeon, Alaska, Delaware, Montana and New Hampshire are five states that do not charge sales tax on car purchases. However, you must know that you cannot purchase a car in one of these states and register it in your home state, in order to avoid sales tax. If you buy a vehicle in Oregon or Montana and get it registered in your home state, you will be charged the applicable sales tax at the time of vehicle registration.