|.. thanks for your visit and commments - more like why are the other parts of the sky NOT blue :-)
There were scattered clouds about in the sky (quite usual for the weather in London). The warm colour you see in other parts of the sky is the result of the glare of the city lights reflected in the low lying cloud. The blue area just happened to be without cloud cover at the time of the exposure - And I have exposed here for 45.7 seconds .. hence you can see the colour of the sky behind the clouds coming through.
In other words the sky is almost entirely covered by cloud except the part(s) in blue.
Hope I made sense :-)
have a pleasant weekend.
Regents Canal IV (186) *