XBOX ONE ONLY
How to buy from Xbox Store US
You need a couple of things to buy from the US Store: a US address and your regular credit card (like VISA).
With these things in hand, follow these instructions:
On your Xbox One go to Settings>System>Language & Location and change your Location to United States and press reboot.
Go to Settings>Payment & Billing and Add credit card Enter your name, your card, and the address provided above (or, as stated before, the address from your Borderlinx account)
Buy stuff from the Store (Optional)
Change back region to whatever it was before, don't worry, you can change as often as you want and your billing info is saved for the next time you want to switch to US Store and buy something.
To buy from UK Store, follow the above instructions but use a UK address.
Cool website to compare prices from different countries: