Under an EU directive of 2002, which states that ‘Summer Time’ will be observed between the last Sundays in March and October, clocks in UK revert to British Summer Time on 31 March; changes either way occur at 0100 Coordinated Universal Time (UTC).
GMT, based on the angular position of the Earth around its axis, and UTC, based on over 400 atomic clocks with leap seconds added, should never differ by more than 0.9 seconds.
So UK leaves its ‘natural’ time, centred on the Greenwich meridian and in the 15º longitude NATO time-zone ‘Zulu’ (Z), used in international travel, timing and, in the past, astro calculations, and moves to ‘Alpha’ (A). The 24 lettered time-zones can be found at World Time Zones
‘Daylight Saving Time’ in Canada and USA is not synchronised with Europe; in 2019 it came into force on 10 March and and will finish on 3 November.
In meteorological terms, Summer in the northern hemisphere will start on 1 June for 3 months. But, astronomically, Summer starts at the Summer Solstice, when the Sun reaches its most northerly latitude of 23º 26.1’ at 1554 UTC on 21 June.
The European Parliament has stated that the adoption of Summer Time in Europe will cease in October 2021. Whether UK complies and, if so, which time zone (zulu or alpha) becomes permanent, remains to be seen. Details are available from the European Parliament