I bought my wilkinson trem from Warmoth with the body and neck. I did think about sourcing the trem in the UK instead, but if you buy the trem from Warmoth they will install the studs for you, which I thought was worth doing as it was the first time I'd put a guitar together and I wasn't sure about my abilities.
Also, I didn't get charged any import duty, which was a nice surprise. I don't know whether they just forgot, or what, but I was very happy. This was about five years ago though, so maybe they are more on the ball now.