1919.
https://www.netweather.tv/forum/topic/32640-heatwave-then-snow-september-1919/
Bear in mind that depending on the upper atmospheric profile, it's more than possible to get snow at 850s of zero, so a quick glance at a chart showing 0 or -1 shouldn't be immediately written-off wrt snow.
Sadly as there were no radiosondes in the 19th century we're stuck with rough guesses as to the uppers, even at 850. I wouldn't rule out reports of snow in London in September either, TBH, as we know that things were a fair bit colder back then. Whereas today it'd be almost impossible, back then in a different era it wouldn't have been quite as hard. (Or, imagining a bell curve, the "cold enough for snow in September in London" part will have shifted from say 1% to 0.01%).
Sure I'm not ruling out the 1885 event but I just think its unlikely given the synoptics were ideal for soft hail and that most meterologists acknoledge that soft hail and snow were very easily confused back then. Also I wouldn't immediately dismiss a snow event based on -1C uppers, but the conditions for snow occuring in those uppers was not imo met in the 1885 event. Its just too easily explained away as soft hail.
The 1919 event though is much more convincing. Occured during the night, A little feature with impressively cold air inside it. I'd definitely say this is the one to beat. Will look into archvied reports from September 20th 1919.
Twitter: @QuantumOverlord (general), @MedicaneWatch (medicane/TC stuff)
2023/2024 Snow days (approx 850hpa temp):
29/11 (-6), 30/11 (-6), 02/12 (-5), 03/12 (-5), 04/12 (-3), 16/01 (-3), 18/01 (-8), 08/02 (-5)
Total: 8 days with snow/sleet falling.
2022/2023 Snow days (approx 850hpa temp):
18/12 (-1), 06/03 (-6), 08/03 (-8), 09/03 (-6), 10/03 (-8), 11/03 (-5), 14/03 (-6)
Total: 7 days with snow/sleet falling.
2021/2022 Snow days (approx 850hpa temp):
26/11 (-5), 27/11 (-7), 28/11 (-6), 02/12 (-6), 06/01 (-5), 07/01 (-6), 06/02 (-5), 19/02 (-5), 24/02 (-7), 30/03 (-7), 31/03 (-8), 01/04 (-8)
Total: 12 days with snow/sleet falling.