Astronomically, Summer finishes at the Autumnal Equinox.
This occurs when the Sun passes over the equator into the southern hemisphere - at 0203 BST (0103 GMT/UTC) on 23 September.
Astronomically, Autumn will run until the Winter Solstice - on 21 December. Then Winter will last until the Vernal Equinox, when the Sun passes back into the northern hemisphere - on 20 March 2023.
But in meteorological terms, Autumn is considered to be the months of September, October and November, with the other seasons following in 3-month periods.
And British Summer Time (BST) will cease at 0200 BST on 30 October, in compliance with an EU directive since 2002, which states that ‘Summer Time’ will be observed between the last Sundays in March and October, clocks to change at 0100 UTC. Currently, UK is continuing to observe that directive.
What I don't understand is why you say astronomical autumn STARTS on the equinox - surely atronomically the equinox is the middle of Autumn and correspondingly the winter soltice should be the middle of astronomical winter and the summer solstice the middle of atronomical summer. Not only does this feel far more logical to me, but it also leads to the expected behaviour that the meteorological seasons lag the astronomical ones, exactly as my engineering training leads me to expect. Fortunately meteorological seasons are far more "human" being related to the way we experience weather on earth.
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