Unable to run Email Summaries feature of Apple Intelligence on my MacBook Pro

The much vaunted Email Summaries feature of Apple Intelligence on macOS (15.2, for UK users) refuses to work. "Unable to summarise this message" seems to be the blanket message for every message. Restart, reboot, it makes no difference.

If you're in the UK or among expanded regions now (supposedly) supported in the 15.2 update, let me know if you have the same issue and if you've manged to resolve it.

(I don't remember the last time an Apple feature actually worked. It just doesn't work.)

[Re-Titled by Moderator]