Taxes

Well it depends what state you live in...but you say you are in the UK. If this is true, I recommend that you purchase content from the GB store.

http://marketplace.xbox.com/en-gb/

Taxes vary from place to place. You'd have to check how much tax is in your area.

It will depend on what state you are or region you should try to see if they charge you taxes

It is regional, and in some regions it can be more specific, such as in the US some states require digital sales tax while others do not, so it can vary quite a bit. If you initiate the purcahse, prior to the last confirm, it should mention if there is
tax included and you can see it from there with the full price of the purcahse. You can check that and decide before full confirmation if you want to purcahse the title or not.

That will depend solely on where you live. Some countries charge sales tax on digital purchases, while others do not. Even in a single country, some regions charge that tax, and others do not. Your best bet is to contact your country/region's department
of revenue or taxation and ask if they charge sales taxes on such purchases.

That said, if you attempt the purchase with your gift card and it asks for another payment method to pay off the rest, that would answer your question, as well. You can always back out of the purchase before you confirm it, of course.