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

Yes. Because of daylight saving time the gap moves between +4h and +5h.

When is the best time to call London from Karachi?

Between 13:00 and 17:00 in Karachi, which is 09:00 to 13:00 in London.

Karachi is 4h ahead of London. This gap is not fixed: daylight saving time moves it between +4h and +5h during the year.

Karachi

Pakistan

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London

United Kingdom

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A 9-to-5 day in each city overlaps for 4h. That is 13:00–17:00 in Karachi, which is 09:00–13:00 in London.

City	Time zone	UTC offset
Karachi · Pakistan	Asia/Karachi	UTC+5
London · United Kingdom	Europe/London	UTC+1

What is the time difference between Karachi and London?: Karachi is 4h ahead of London. This gap is not fixed: daylight saving time moves it between +4h and +5h during the year.
Does the time difference between Karachi and London change during the year?: Yes. Because of daylight saving time the gap moves between +4h and +5h.
When is the best time to call London from Karachi?: Between 13:00 and 17:00 in Karachi, which is 09:00 to 13:00 in London.