Roise, roughly, I gather VAT is payable on anything over £18 that's anywhere outside the EU. From memory I gather this is going to go down to £15, sometime this year, if it already hasn't! VAT is also not payable for any items purchased under £40, which are classed as a family gift. Septate to VAT, I think Customs Duty is payable on most items over £135, although the rates vary depending on the type of item, imported.
Some of this may have changed slightly this year. Just looking, I think VAT itself is not always payable, there are a number of factors involved, however, Import VAT is charged on nearly all imports by Customs and Excise, regrettably.