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

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

When is the best time to call Singapore from London?

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

London is 7h behind Singapore. This gap is not fixed: daylight saving time moves it between −8h and −7h during the year.

London

United Kingdom

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Singapore

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

City	Time zone	UTC offset
London · United Kingdom	Europe/London	UTC+1
Singapore · Singapore	Asia/Singapore	UTC+8

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