Don't confuse VAT and import duty - they are entirely different things.
When you import, basically, you pay for the cost of the goods plus the cost of shipping. When it arrives in the UK, you pay import duty as a percentage of the total cost of the goods and shipping. This can vary from 0% through to 15%+ depending upon the product. You can get these exact rates by calling HMCE.
You then also have to pay agents fees for clearing the goods through customs for you and one or two other small charges.
The killer is that you then must pay 17.5% VAT on EVERYTHING so far including all the fees, shipping, import duty etc. Of course if you are VAT registered then you claim this back but you still need to make provision in your cashflow to cover it.