London is indeed rather ill prepared for snow, I remember a near total transport shutdown a few years ago due to only a few inches of snow.
Many of our trains are electric and these do not cope well with even modest snow, some use a live third rail to supply current to the train, these are especialy vulnerable.

I would assure those who have not seen it for themselves, that not just one, but dozens of snowflakes landed in central London.

Temperature at present is 2 degrees (centigrade) which is lower than average but not remarkable.
Last night it reached minus 4, again a little below average but not remarkable.

I have however resorted to wearing long underwear, which is not the norm.
Similar temperatures and a little more snow are expected.