Astronomically, Spring starts at the Vernal Equinox, when the Sun crosses the equatorial into the northern hemisphere - at 0349 UTC (~GMT) on 20 March.
Astronomical Summer then starts at the Summer Solstice - 2144 UTC on 20 June, when the Sun is at its highest northerly latitude of 23º26’; it returns to the southern hemisphere at the Autumnal Equinox at 1330 UTC on 22 September.
From 2002, the EU has stipulated that all member states should adopt ’Summer Time’ between the last Sundays in March and October - the clocks to change at 0100 UTC. So, this year BST will apply from 29 March to 25 October, even though we are no longer within the EU . .