Ha, I just got stung for VAT yesterday! $50 in charges in a $250 lens bought here, declared value $80.
The limit is £38 pounds for gifts and £18 or £19 for commercial items. If you use Global Express Priority, I think, the package goes through Parcelforce. Parcelforce ALWAYS seem to get you stung for Customs; of the last three items they've delivered, every single one has attracted VAT, plus an extra £10 or £20 fixed fee.
If the item is less than £150, I would personally ask for it to be sent regular airmail. This is, ironically, quicker, and never seems to attract duty - I note this is how Stephen Gandy sends all his gear. If the item is £200, I would probably ask for it to be sent priority, but declare the value at $70, which should fall under the limit. If it were more, I would personally declare the full value and reclaim the VAT, which fortunately I can do.
In some cases, senders will allow you to put a different value for insurance, and for declared customs value. there is a logic to this - i might have bought a camera part for $20 s/h, but to buy it new would cost $100. When I bought a lens from D30 Gaijin at photo.net he managed to do this, but others didn't.