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

Yes. Because of daylight saving time the gap moves between +6h and +7h.

When is the best time to call London from Bangkok?

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

Bangkok is 6h ahead of London. This gap is not fixed: daylight saving time moves it between +6h and +7h during the year.

Bangkok

Thailand

--:--:--

London

United Kingdom

--:--:--

A 9-to-5 day in each city overlaps for 2h. That is 15:00–17:00 in Bangkok, which is 09:00–11:00 in London.

City	Time zone	UTC offset
Bangkok · Thailand	Asia/Bangkok	UTC+7
London · United Kingdom	Europe/London	UTC+1

What is the time difference between Bangkok and London?: Bangkok is 6h ahead of London. This gap is not fixed: daylight saving time moves it between +6h and +7h during the year.
Does the time difference between Bangkok and London change during the year?: Yes. Because of daylight saving time the gap moves between +6h and +7h.
When is the best time to call London from Bangkok?: Between 15:00 and 17:00 in Bangkok, which is 09:00 to 11:00 in London.