In meteorological terms, Winter in the northern hemisphere covers the months of December, January and February.
But, astronomically, Winter starts at the Winter Solstice, when the Sun reaches its furthest point south of 23º26.1’ - this year at 0419 UTC (~GMT) on 22 December. It will be followed by Spring, starting at the Vernal Equinox, when the Sun re-crosses the equator at 0349 UTC on 20 March 2020.
Since 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, assuming we stay aligned with Europe, in 2020 BST will apply from 29 March to 25 October. How long UK complies with this may be open to question.