They tried to setup one around me for a new 20MPH zone, but I've never
seen it operating in the 20MPH zone, only in the older 30MPH areas.
I suspect they never finished off the 20MPH signage to legal
requirements (in particular, the repeaters or traffic calming measures
required were not implemented, just 20's painted on the road, which I
don't think count for anything).

pedantry_watch
--
Robin
reply to address is (meant to be) valid