Normally, this PC is left on 24/7 (it's doing stuff). A couple of very recent updates have meant that it was been rebooted twice yesterday evening - once for the new kernel and once for.. something else that was updated after the first one.
Following those, the keyboard was US English.
Only UK English was previously in my preferences. Afterwards, only US English was.
This applied to Preferences / Keyboard and in iBus.
I have no idea why this was changed.