I almost bought a system from the U.S.A. from a legit Mac seller. However, I realized that it would cost nearly the same as if I bought it from the Apple Canada store, since you definitely have to pay all the taxes when it reaches your house. You can have your seller label it as a gift, but be careful with purchases as large as a laptop. If someone catches your "gift" at the border, you will have to pay huge fees (since you can only label a package a "gift" up to a certain price). It's more obvious that your item costs more than the maximum gift price with a computer since it's heavy and large.
Basically, if your seller is from the UK him/herself then you shouldn't have much to worry about (and maybe you can even travel out to wherever they are to save shipping costs). However, if they are located somewhere else you will have to pay taxes unless you have the seller mark the package as a "gift", which you will have to do at your own risk.