I always thought that the Hampstead storm was what the Americans (&
Australians) defined as a "severe pulse" storm - that is, a severe single
cell without rotation.


That fits the bill rather well. The storm broke out at the southern end of
a line of convergence of surface winds, the synoptic situation being a very
weak southerly. Definitely not a supercell.

There was another (severe AND tornadic) storm in London in the 1950's but
I've got no further info on this one just some faded memories of some pathe
newsreel footage of the time.

Very likely 7 Dec 1954. There was an unstable cyclonic south-westerly
and the storm did a fair amount of damage but it seems to be to have been a
long way from being a supercell. Maybe more like the series of severe storms
that flooded parts of Sussex on 11/12 October 2000. A warm moist surface (the
Channel), an unstable atmosphere and a good deal of local cyclonic and coastal
convergence. Tremendous rainfall but again, not supercells.

Tudor Hughes, Warlingham, Surrey.