Hey, just read this. Apologies if this is common knowlegde but are you aware that there is a tariff code (entered in the tarriff number box) for UK customs whereby all limited edition printed art is eligible for a 5% VAT rate, rather than the 20% usually charged on imports?
http://tinyurl.com/bcxt8z5 - section 11:
I recently bought a print of Dru Struzan's The Thing Mondo and was charged 5% rather than 20% as the seller used the code above.
So, even if a seller isn't 'customs friendly' you could request they use this code. And if not, you can claim it back retrospectively.
Of course, still better to have under $15 declared