There is no precise date. The UK summer is a ghost—sometimes it lingers until October, other times it packs its bags on August 10th and leaves without saying goodbye. The only certainty is that it will end without warning, likely on a Tuesday afternoon when you’re in the middle of doing nothing important.
My advice? Keep a light jacket in the car. Enjoy the sunny September days as a bonus round. And when you see that first conker on the pavement, just nod. It’s over, pal. See you next year, summer. We’ll pretend you were great.