Astronomically, Spring starts at the Vernal Equinox, when the Sun crosses the equatorial into the northern hemisphere - at 2158 UTC (~GMT) on 20 March.
Astronomical Summer then starts at the Summer Solstice - 1554 UTC on 21 June, when the Sun is at its highest northerly latitude of 23º26’; it returns to the southern hemisphere at the Autumnal Equinox at 0750 UTC on 23 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 31 March to 27 October, irrespective of the Brexit situation . .