To be able to use a Battle.net Gift Card, you require the Battle.net Authenticator in your account. This code can only be activated with an UK account!
With the Battle.net credit, you can purchase a multitude of games, digital items in Blizzard Games (e.g. pets and mounts in World of Warcraft or card packs in Hearthstone) and other services (like additional gametime in WoW). An upgrade from the standard edition of a game to the respective Digital Deluxe Edition is also possible with your Battle.net credit.