CR123A battery £1.99 in screwfix. Edit: Both of mine arrived today (separate orders via Netherlands post - free postage). Each marked 9 Euro and no customs to pay. Edit2: I have tested all the SmartThings device handlers and this is the best and only one to correctly report battery level correctly. (https://github.com/bspranger/Xiaomi/blob/master/devicetypes/bspranger/xiaomi-mijia-honeywell-fire-detector.src/xiaomi-mijia-honeywell-fire-detector.groovy)
The DH I used is: bspranger : Xiaomi Mijia Honeywell Fire Detector I tested it with a burning match when I first installed it and it worked fine. It hasnt gone off since, but it's upstairs so doesnt really get exposed to any kitchen smoke. I have it as a cheap addition to a Nest Protect.
Ordered thanks OP
Poland, and a couple of hubs in Germany, are part of the China Belt and Road project to push goods around more cheaply. If you choose the rail shipping from Gearbest, you'll notice it goes through Kazakhstan, Russia, Belarus, to Poland. But Gearbest have also used warehousing in the UK, and Spain, probably some other places like Luxembourg or Liechtenstein.
Dispatched from the EU normally means they send it on a train into Poland and it gets 'dispatched' from there. Wonder why they choose Poland? Got to be something dodgy going on given GearBest's track record. Can't help but notice that Belarus is next door....