The Wokingham storm of July 1959 was certainly a supercell, as Les says,
as was probably the even more dramatic Horsham hailstorm of 5 Sept 1958
(unusually late in the year), with cricket-ball sized hail. It is difficult to
see how the Hampstead storm could have been a supercell as it was stationary
and the wind speed at all levels was quite low, as far as I know. One of the
requirements for a supercell is considerable wind shear to maintain the
structure and this was certainly present in the Wokingham storm, as it moved
NE'wards against the surface flow. The Horsham storm is a less obvious case.
As one who witnessed it (though the large hail had ceased) I still reckon it
was the best storm I have seen. Cloud tops reached 48,000 to 52,000 feet and
the lightning was unbelievable. Spoilt for life at the age of 15!

Tudor Hughes, Warlingham, Surrey