In article ,
John Hall writes:
the three coldest January days in London on record are, in
chronological order:

8th January, 1841: max of -8.3C at Greenwich
4th January, 1867: max of -8.3C at Camden Square
12th January, 1987: max of "reportedly" -9.1C at Warlingham and -8C
"across large parts of the south"

One other point. The two 19th century figures won't have been obtained
from a thermometer in a Stevenson Screen. However if, as is likely, they
occurred in a strong easterly wind, then I wouldn't expect that to make
any significant difference. (Not that I'm an expert.)
--
John Hall
"Sir, I have found you an argument;
but I am not obliged to find you an understanding."
Dr Samuel Johnson (1709-1784)