If you use Windows 10, you likely already have a Microsoft account. You go to minecraft.net, select login, then 'Sign in with Microsoft'. Then you need to click 'get Minecraft', select the edition/platform, and there should be an option to use a gift code as payment. Once you have the game, there will be a download link.
Edit: at that link it states "available only to US customers", so if he can't buy it for you from UK, might need to look for some other online place selling those cards.