I walked in to a US Apple store and explained the problem. The Genius person agreed there was a fault and then started tapping away at their computer. He umm'ed and ahh'ed and called over a manager.
At this point I was thinking they were going to reject the warranty repair for some reason, but the manager ended up apologizing that they didn't have any UK-style MacBook replacement keyboards in stock, and they couldn't work out how to order the part in on their US system.
As it happens I had moved to the US, so getting a US keyboard was actually a bonus. I told them I was happy with a US keyboard, and 10 minutes later I was leaving the store with it installed in my MacBook.