Does the time difference between Hong Kong and London change during the year?

Yes. Because of daylight saving time the gap moves between +7h and +8h.

When is the best time to call London from Hong Kong?

Between 16:00 and 17:00 in Hong Kong, which is 09:00 to 10:00 in London.

Hong Kong is 7h ahead of London. This gap is not fixed: daylight saving time moves it between +7h and +8h during the year.

Hong Kong

Hong Kong SAR China

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London

United Kingdom

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A 9-to-5 day in each city overlaps for 1h. That is 16:00–17:00 in Hong Kong, which is 09:00–10:00 in London.

City	Time zone	UTC offset
Hong Kong · Hong Kong SAR China	Asia/Hong_Kong	UTC+8
London · United Kingdom	Europe/London	UTC+1

What is the time difference between Hong Kong and London?: Hong Kong is 7h ahead of London. This gap is not fixed: daylight saving time moves it between +7h and +8h during the year.
Does the time difference between Hong Kong and London change during the year?: Yes. Because of daylight saving time the gap moves between +7h and +8h.
When is the best time to call London from Hong Kong?: Between 16:00 and 17:00 in Hong Kong, which is 09:00 to 10:00 in London.