Does the time difference between Cairo and London change during the year?

Yes. Because of daylight saving time the gap moves between +1h and +2h.

When is the best time to call London from Cairo?

Between 11:00 and 17:00 in Cairo, which is 09:00 to 15:00 in London.

Cairo is 2h ahead of London. This gap is not fixed: daylight saving time moves it between +1h and +2h during the year.

Cairo

Egypt

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London

United Kingdom

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A 9-to-5 day in each city overlaps for 6h. That is 11:00–17:00 in Cairo, which is 09:00–15:00 in London.

City	Time zone	UTC offset
Cairo · Egypt	Africa/Cairo	UTC+3
London · United Kingdom	Europe/London	UTC+1

What is the time difference between Cairo and London?: Cairo is 2h ahead of London. This gap is not fixed: daylight saving time moves it between +1h and +2h during the year.
Does the time difference between Cairo and London change during the year?: Yes. Because of daylight saving time the gap moves between +1h and +2h.
When is the best time to call London from Cairo?: Between 11:00 and 17:00 in Cairo, which is 09:00 to 15:00 in London.