Thanks. After checking with `defaults read -g AppleLanguages` I discovered that I didn't have British English set as my language at all, but only the region as United Kingdom. Once I changed that everything was fine, as you might expect.
I'm getting dates in en_US style even though I should be getting en_UK ones; `locale` in the terminal shows en_GB and the main language setting is United Kingdom. Is there something that I need to do to get the workflow to pick this up?