I was sitting in my garden as sunset approached and saw a small bright moving spot about 5 or 10° above the horizon at most. The light appeared brighter than in the photo.
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I looked at FlightRadar 24 and there were a few aircraft in the direction of London, about 55 miles away, either heading for Heathrow or departing so I expect it was the sun reflecting off one of them. The small spot to the left could be one of the winglets catching the sun.
I'm disappointed there's a likely explanation.