Dave
It can be the luck of the draw, depending how the shipment is sent to you, generally if it arrives via couriers like UPS you will pay import duty (about 2%) and VAT on your purchases, they will also charge VAT on the cost of shipping. If it comes by normal mail you will more than likely get away with it. Either way it can still work out cheaper, you have to treat each case individually.
For some items I have saved a fortune by buying from the US and paying the duty, others it just isn't worth it.
One thing I would say is the service I have received from the US has been incredible compared to what we get in the UK.
I placed an order with Cabelas on a Monday afternoon, I could track the package all the way over the UPS web site and see when it was on its way to me from the airport and it arrived Friday morning.
Lee