British Double Summer Time was a Second World War-era extension of the existing Summer Time Act. The existing system involved setting the clocks an hour forward on the last Sunday of March and setting them an hour back on the last Sunday of October, to take maximum advantage of daylight hours.
In winter 1940, Britain did not set the clocks back an hour, and retained the hour's advance of the summer. The clocks were then set forward another hour the following summer, meaning they were two hours ahead of GMT.
Regular Greenwich Mean Time was restored at the end of summer 1945.