I think they're averaging throughout the year, using the number of "Mean Annual Sunshine Hours" * "Percent Possible Sunshine", which would account for average c

I think they're averaging throughout the year, using the number of "Mean Annual Sunshine Hours" * "Percent Possible Sunshine", which would account for average cloud cover and such. I'm sure that for a few hours during the summer, the solar irradiance in London is probably 90% of what it is at the equator. That doesn't help most of the year.