Astronomically, Spring starts at the Vernal Equinox, when the Sun crosses the equatorial into the northern hemisphere - at 0306 UTC (~GMT) on 20 March. Astronomical Summer then starts at the Summer Solstice - 2050 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 1243 UTC on 22 September.

In 2002, the EU 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 (Easter Sunday) to 27 October, even though we are no longer within the EU.

Further details from 

https://www.timeanddate.com/calendar/seasons.html