I have bought the cards in USD, and used them in Canada. I believe you can use them worldwide, starbucks has an automatic currency converter. Not sure if you can redeem the cards ONLINE in that currency though, I always redeem them in store.
Thanks for the reply. You can always ask a starbucks employee. If you want, I can sell a smaller amount for you to try ($5 for $2.5 in BTC).
Found this:
https://answers.yahoo.com/question/index?qid=20130624225617AAZLfEJStarbucks Cards (including Starbucks Card eGifts) are accepted at most Starbucks locations in North America, including airport and grocery locations. Unfortunately, Starbucks Cards are not accepted in Germany at this time. Starbucks Cards can be used interchangeably at most stores in Australia, Hong Kong, Ireland, Mexico, the United Kingdom, and Thailand. Starbucks Cards issued in Greece, Japan, Korea, Russia, Malaysia, Singapore, and Taiwan may only be used in the country from which they are issued.
Also
http://www.starbucks.co.uk/customer-service/faqs/card confirms that UK, USA, Canada, Australia, Hong Kong, Mexico, and the Republic of Ireland is in some kind of alliance.
I'm not in any of those countries to think it's doomed. Thanks anyway and good luck.