John Hall wrote:

No chance of frost fairs on the Thames in London, I'm afraid. The
demolition of the old London Bridge, which had very narrow arches
which obstructed the flow, and the embankment of the river have done
for them. 1962-3, the third coldest winter in the CET record, showed
that it's pretty much out of the question.

Couldn't decent size plates of ice from up stream, and burgy bits in the
mix, slow the flow enough? I mean solid waste only moves ten miles
downstream a month given the tidal flow and all? Ramsgate sea even froze in
'63!