If you do buy from the States make sure you buy from a reputable dealer, I have just been ripped off buying second hand.
VAT is 17.5% and Import Duty 3.2%, payable on new and second hand goods.
The postage will be between £25-£50 and the shipping company will charge you approx £10.00 handling fee.
All in all not worth the hassle! unless you are buying a top of the range marker.
Go to a good local paintball shop and see what they have. I know you can pick up a good starter marker for around £100.00 at Phoenix Paintball.