add 22.5% of the total value of goods you buy
Also depending on courier you may have to pay about £20 handling charges.
The pound is strong at the moment so it is a good time to buy.
However consider the warranty of the product you are buying. ie. does it only cover you for warranty repair in the country of purchase (some manufacturers stipulate this).
So at the moment say you wanted to buy a new DM4 matrix
They retail here at £1000 from Planet, plus say £10/£15 shipping
So total of £1015, and you get it next day or so...
Buy it say in the states from action village for example.
Cost $1400, plus $75 shipping.
This is equal to £781 (at todays rate of $1.88 to the £)
Add your 22.5% to this in Vat and import tax. £956. Quite a wait for it to arrive, and maybe a handling charge, £976.
What looks like a bargain soon becomes a small saving of about £40, and you lose the luxury of going to the nice techs where you bought it here and saying 'oi me two week old gun is broke can you fix it please'
Some people will argue different, but personally unless it is an amazing bargain it often isn't worth the time/effort you put in for what you save