The current rule
The United Kingdom moves to British Summer Time at 1:00 a.m. GMT on the last Sunday in March, and returns to Greenwich Mean Time at 2:00 a.m. BST on the last Sunday in October. The UK is on GMT — that is, UTC+00:00 — through the winter, and on UTC+01:00 through the summer. The country changes at the same instant as the rest of Europe, since the EU rule is defined in UTC.
GMT is not the same as UK time
This trips up a great deal of software. Greenwich Mean Time is a fixed offset of UTC+00:00; UK local time is GMT in winter and BST in summer. Labelling a summer meeting "10am GMT" when you mean 10am in London is wrong by an hour, and it is one of the most common scheduling errors in international business. The safe habit is to say "London time" and let the software resolve it.
British Double Summer Time and the 1968 experiment
During the Second World War Britain ran Double Summer Time, two hours ahead of GMT in summer, to extend working daylight. Between 1968 and 1971 the country tried staying on UTC+01:00 year round under the British Standard Time experiment. It was abandoned after three years, largely because of dark winter mornings in Scotland and northern England, an argument that still shapes the debate today.