Rough guide:
1. Download and install Connectify
2. Download and install Speedify
3. Open Speedify, click through the startup screens
5. Click the little cog under 'Upgrade' set the location to Turkey, whichever region next, then the 'X'. Speedify is now setup and should be connected to a Turkish server
6. Open Connectify, when starting the hotspot make sure it's using 'Speedify' under 'Internet to share' (there's a drop down for which network connection to use as the hotspot). PC is now ready to use as a hotspot via the Turkish VPN. You can check this by looking under 'Privacy monitor' in Connectify, it should have the Turkish location
7. On your Xbox set the location to Turkey, restart the console
8. Connect the Xbox to the Connectify hotspot
9. Browse the game you want in the store, when you come to buy add any Turkish address for the payment (I used the UK embassy), add your card details
10. Buy, profit.
11. Set the Xbox region back to UK, restart, reset your network settings if using WiFi.
Worked perfectly for me earlier, using my MBNA credit card.