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

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

When is the best time to call Toronto from London?

Between 14:00 and 17:00 in London, which is 09:00 to 12:00 in Toronto.

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

London

United Kingdom

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Toronto

Canada

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A 9-to-5 day in each city overlaps for 3h. That is 14:00–17:00 in London, which is 09:00–12:00 in Toronto.

City	Time zone	UTC offset
London · United Kingdom	Europe/London	UTC+1
Toronto · Canada	America/Toronto	UTC−4

What is the time difference between London and Toronto?: London is 5h ahead of Toronto. This gap is not fixed: daylight saving time moves it between +4h and +5h during the year.
Does the time difference between London and Toronto change during the year?: Yes. Because of daylight saving time the gap moves between +4h and +5h.
When is the best time to call Toronto from London?: Between 14:00 and 17:00 in London, which is 09:00 to 12:00 in Toronto.