I really doubt this idea, and I wouldn't be surprised if the truth was completely the opposite. Since people in high-latitude environments are left with relatively few days with pleasant weather, they cannot efficiently schedule their fun outdoor activities. Here in the northern Midwest, the first nice day of spring (or the occasional unseasonably warm day in fall) often has many people taking time off on a weekday, and the people who do show up can't get much done with so many unplanned absences. Meetings are cancelled, etc. If the weather was not a factor, people could schedule their fun time more efficiently (i.e. weekends, slow periods at work, etc.)

In this grey lowlight world of English weather, with the added factor that by and large interaction between anglophones is mostly linguistic, it is not admirable that many come to office even on Saturdays and Sundays, and also stay late during weekdays. Basically, where else would they go?

This is entirely wrong. The bleak winter season doesn't make people more likely to work, it makes them more likely to want to curl up inside and eat. Basically Seasonal Affective Disorder seems to be an adaptation, encouraging people toward lethargy and energy conservation during winter.