I think your UK router will work perfectly with the EU firmware, and if it doesn't, it may have some problems, but you may be able to flash it again with the UK firmware, so there's no problem.
Also, the warranty only voids if your router gets bricked with an unsupported firmware installed. If you flash it back with the UK firmware, you will still have your warranty (at least I was told that when asking about the 1.11 b39 firmware in this forum)