Just an anecdotal observation, as a Rainbow, Brownie & Guide Leader for 10+ years (stopped in 2019 due to illness, though it’s glorious having my evenings and weekends back!), we did many events for the girls in September, and with the exception of a couple of events (fortunately indoors) where we’ve had torrential rain, it’s always been fine weather.
I’ve also done umpteen camps in September & again, from dawn til dusk it’s usually been in the mid teens to early twenties (I think we had frost once in the morning of a later September camp at the end of the month), but that’s an anomaly in my experience.
In fact, we’ve had more calmer & perfect camping days in September compared to April & May camps, where in one day we had frost in the morning, mid 20s by lunch, followed by a glorious thunderstorm & 2in of hail gathered up the sides of the tents!
As for the longer summer camps in August, we’ve had one in 2012 where it seemed to be permanently grey & drizzled for a week, in later years scorching days where, as first aider, I’d spend most of the day handing out sunscreen & dealing with the older teens who think they’re invincible getting sunstroke & sunburn.
As for events & camps in July, I can remember one where we had torrential rain & storms where we’ve been flooded out! And the times we’ve been on the verge of cancelling or actually cancelled mid event due to bad weather has always been in June & July.
September was a favourite time to camp as it’s very much a transitional month from summer. Mostly fair & bright, just cool enough to sleep well, the odd shower but no high winds (lost £1k in tents in 30 seconds in a May Bank Holiday camp once, despite making them more secure than Wormwood Scrubs when we pitched them. There was definitely cloud rotation, on top of a hill with a great view by chance of incoming weather from all sides, we were just unlucky to be caught in the non tornado but rotational wind).
Apologies for a rambling anecdote, it’s been one of those days.
Welwyn Garden City, Good ol' Hertfordshire.