Another suggestion: National book token? It'll encourage the winner to spend the prize on educational material without endorsing one business over another.
NBT shouldn't be a problem for a winner residing outside the UK, because the online shops of most major UK bookstores offer international delivery. In fact it'll encourage overseas winners to spend the NBT on books they can't buy in their home countries, thereby increasing the potential for them to contribute to Wikimedia projects on a UK-related topic.
Deryck