The weather in London is usually noticeably warmer than in Northen Europe. In winter, there are sometimes minus degrees (Celsius of course) and snow, but not significantly. Observe older residential buildings - many have water pipes running from outside the house.
But to see the famous London fogs, you would have had to have been born a hundred years earlier...

The coldest temperature in London was recorded in 1962 (in January): -16.1oC.
The highest temperature was in 2003 during a heat wave: 38.1oC.