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

Yes. Because of daylight saving time the gap moves between +2h and +3h.

When is the best time to call London from Istanbul?

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

Istanbul is 2h ahead of London. This gap is not fixed: daylight saving time moves it between +2h and +3h during the year.

Istanbul

Türkiye

--:--:--

London

United Kingdom

--:--:--

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

City	Time zone	UTC offset
Istanbul · Türkiye	Europe/Istanbul	UTC+3
London · United Kingdom	Europe/London	UTC+1

What is the time difference between Istanbul and London?: Istanbul is 2h ahead of London. This gap is not fixed: daylight saving time moves it between +2h and +3h during the year.
Does the time difference between Istanbul and London change during the year?: Yes. Because of daylight saving time the gap moves between +2h and +3h.
When is the best time to call London from Istanbul?: Between 11:00 and 17:00 in Istanbul, which is 09:00 to 15:00 in London.