Daylight Saving occurs on very specific dates and as such it is not difficult to manufacture a device which automatically switches from GMT to BST and back again.
British Summer Time is determined by an Act of Parliament: the Summer Time Act, 1972. This Act enables an Order in Council to specify exactly when the changes will occur. Such "Summer Time Orders" have, in the past, usually defined BST for the following three or four years (for instance, the Summer Time Order, 1997, which gave the dates for 1998, 1999, 2000 and 2001).
However, since the Summer Time Order, 1994 (that is, in 1994 and 1997) the dates have been those determined by the EC directive on daylight saving, thus formally synchronised our summer time changes with Europe (which is the last Sunday in March and the last Sunday in October).
The 2002 Order repeals both the ability of the 1972 Act to vary British Summer Time and the ability to introduce Double Summer Time. It formally adopts the EC Directive and gives the formula:
(2) The period of summer time for the purposes of this Act is the period beginning at one o'clock, Greenwich mean time, in the morning of the last Sunday in March and ending at one o'clock, Greenwich mean time, in the morning of the last Sunday in October.