I've purchased games from the Brazil store occasionally, but the removal of gift card redemption at checkout on the updated website recently had scared me off. I see the latest advice found on a well known UK deals site now suggests doing transactions on the console itself like this - perhaps I'll try it next time there's a bargain that I'd like:
Go to Xbox console settings and change your LOCATION region to Brazil.
Restart Xbox and head back to the store.
Add Payment Method > Redeem Gift Card (self explanatory)
It might ask for an address, as always Google is your friend.
Complete purchase of game as normal.
Be sure to change LOCATION region back to United Kingdom once purchased.
I didn't try that console based method, after learning there's a much easier procedure for redeeming foreign currency gift cards that still works directly in a browser. At the new store page for a desired game, clicking the gift icon to begin the process of gifting a game permits gift cards to be redeemed at checkout; the gift purchase itself can then be cancelled before completion and the credit used to buy it for yourself instead. So that's a little more fiddly to do than previously, but nevertheless works.