According to Bonacina, the earliest snowfall to have been recorded in London was on the 25th of September 1885.

From the Times of 26th September 1885

Other reports from this month.

Langton Herring: The great cold from 25th to 28th was very remarkable and sleet and hail fell in many places.
Culford: On the 26th, the frost cut down most of the bedding and other tender plants.
Seathwaite: First snow on hills on 23rd

Minima
Camden Square: 0.9C (27th)
Hitchin: -0.5C (28th)
Bury St Edmunds: -2.2C (26th)
Orleton: -2.8C (28th)
Leicester: -0.2C (28th)
Braemar: -3.3C (27th)
Dublin: 1.3C (27th)
Omagh: -1.1C (26th)

Interesting article in that edition of the Times about the shrinking Alpine glaciers

I witnessed the snow event of 6 October 1974, when I was at teacher training college in Cockfosters at the very outer reach of the Piccadilly Line - so just about in London.

It was a terribly cold, wet morning, with definite big flakes of wet snow mixed with the rain, followed by a prodigiously mild autumn. The tabloids were predictably reporting daffodils in flower before Christmas.

I was also in London for the event of 2 June 1975. At Tottenham, N17, I may have seen sleet, but definitely not snow - though my parents in Spalding, Lincs, did have snow that morning, lying briefly on the ground.

The 1885 event sounds borderline to me. Probably hail by today's standards.

Witnessed events from both 06/10/74 and 02/06/75 in Essex, both contained definite snow, and during the latter it settled on some surfaces. Interestingly (as I have said before on other threads), this must make the 74-75 winter season the longest on record by far.

As an addendum, with respect to London's earliest snowfall, there have been many instances when it has snowed just after midnight on the 1st January.

N.