I think the problem is possibly that someone took a figure for an
average speed and assumed that's what everyone always goes at. ICBW but
I've heard 11 or 12 as a CTC (or similar) measure of average speed,

I think that's the speed quoted as an average for a cyclist in London,
often compared to the average speed for other modes. Outside London I'd
expect it to be higher.

Where the 12mph came from in this thread, is when I was in an exchange
with an NCN officer over a path which had come to resemble the North
Sea in January because of tree roots pushing the tarmac up. In one
email I wrote "I don't ride a full suspension mountain bike (as there
are very few mountains in the Fens), so riding along that path even as
slowly as 12mph would be enough to cause me discomfort or even injury."
His reply was "I wouldn't dream of cycling as fast as 12 mph along
there".

So he claims to be a cyclist. He also thinks 12 mph is fast. My grandad
used to cycle faster than that (on a flat straight road) when he was 92
and had two replacement hips, but the NCN officer thinks reasonable
progress on a commuting journey will be made when the top speed is
12mph (ie the bit on the straight path, not the junctions or bends).
It's no wonder the paths are so poorly designed when the self-professed
cyclist officers clearly are not cyclists.