I think the largely horrible spring and then the entirety of ‘high summer’ being trough dominated has contributed to this. Otherwise, a cool-down to something more seasonal might have been more welcome? Mind you, September isn’t usually a cool month generally, especially in the south.
I’ve very much enjoyed this September, am hoping that October has some nice sunny weather too but with some more seasonal mists, fog, and toward the end of the month, frosts.