Limit before being charged import duty and VAT for normal/commercial items is £18.
Limit before being charged import duty and VAT for 'gifts/demos/samples' is around £30 (unsure of the exact figure, go look it up on the GB Customs and Excise website).
Of course, when the item is marked as over that price, its completely at random as to whether you get stuck with a fee or not - the customs officials cannot process every single package. Some couriers are also worse than others (Parcel Force, UPS and DHL seem to stick large 'payment' fees on, even if the duty was only a small amount - they 'paid' the duty on 'your' behalf.. nice racket they've got going on.)
Just gotta think whether you're saving enough for it to be worthwhile, even if you get stuck with a fee (I regulary use HLJ.com for models and stuff, and its always far cheaper, even if I get import duty to pay).