Ask frequent visitors for London’s best-kept secret and many will name September: summer’s warmth lingers, the August crowds evaporate, and the city’s cultural season restarts with genuinely free events.
London Weather in September: The Short Answer
September brings highs around 20°C and lows near 12°C, with about 50mm of rain. The first half of the month is noticeably warmer than the last, and daylight slips from roughly 13.5 hours to 11.5 by month’s end. It is reliably mild — proper coats can wait.
What to Pack
Layers are the whole strategy: t-shirt weather at noon, light-jacket weather by dinner. Add a compact umbrella and you are covered for the month’s showery interludes.
What’s On in September
The Totally Thames festival animates the river all month, Open House Festival unlocks hundreds of normally closed buildings for free, and London Fashion Week brings its circus to town. Museums launch their autumn exhibitions — the calendar thickens noticeably.
Crowds and Prices
The school run resumes and tourist numbers drop within the first week — queues shorten and hotel rates ease off their August peak, especially midweek. It is arguably the year’s best value-to-weather ratio.
Verdict
For the balance of weather, prices and breathing room, September is the smart month — we’d pick it over high summer for most trips. Compare it with August before you book, or see how autumn deepens in October.