About the customs, they will add VAT to the total amount you paid for it by reading your invoice, so if it says $699(448.652 GBP
) then you will get charged £78.50 + £10 storage, but if you have no invoice then they cant set a price to charge, but they will call you asking for the velocity it shoots at, tell them 270FPS and 12lb/si. You will still get charged storage though.
So ask the store you are buying from to send the invoice through the normal post and ask them to write Gift on the box. It would also be a good idea to get them to put a sheet in the box that says the velocity in FPS and lb/si.