why do you think this is related to the change from winter time to summer time? For HEL-LHR the flight leaves Helsinki about 50mins later in the summer than in the winter. But this is not related to the change of the clocks and the change of schedule doesn't happen at the time the clocks change. And the time difference finland-UK is always two hours year round anyway.

perhaps in the winter the days are shorter and darker and people wouldn't have much to do in winter evenings. With longer days they might want to stay in Helsinki longer.