I bought a bass for $1400 + $100(shipping) in the USA and imported it to Germany. The customs wanted to me to bring a recipe of the payment when picking it up. I also had to bring about 300€ for 19%VAT + 3.75% custom fees.
The recipe, in my case, was a copy of the paypal payment, as I bought via ebay. I don't know if UK customs act similar, but if so you would have to ask the seller if you can split your payment in 2 parts, one for the customs and one adding the rest. This way you would have a recipe showing a lower price.
EDIT: I forgot: the 19% + 3.75% were taken from the actual price incl.(!) shipping costs..
EDIT2: forgot something else: If the seller is willing to accept split payment tell him to not put a recipe with the full price into the package in which the bass is shipped

At least I had to open the package because the customs wanted to check its contents.