I looked it up... Amazon.co.uk Gift Cards and Gift Vouchers (including products branded as Gift Certificates) ("Gift Cards") may only be redeemed toward the purchase of eligible products on www.amazon.co.uk.
To be able to use a uk gift card you have to use it from the website it was sold from or have to use it it on a website that works for the uk. You cant use it to ship to the us unless you talk with the seller privately and can convince them to ship it to you which isnt likely to happen.