Here in Molesey, on the outer fringes of London, semi-urban area but in a reception area for cold air drainage, sunny all afternoon but with a falling temp after a max of 2.8C....but...

4.30pm -1.6C.
5.45pm - still clear, no cloud. Temp -2.5C
7pm up to -1.0C and foggy.

It goes to show that you cannot cover the temperature over the SE / London with one figure on a national forecast.

Julian