If you are buying an RTR, even from a US company, it is most likely made in China (even if OEM).
You can most likely buy the direct from China or even other suppliers in Europe. If you can buy it in the EU you will escape a lot of the problems mentioned in earlier posts.
Also, anything imported into UK, you not only have to pay any duty, there is also a handling charge for the customs paperwork.
I am not sure of the current status of duty paid items imported from the EU.