SE England has a comparatively wonderful climate to the rest of the UK (well at least to Wales and Scotland). I'm very jealous of the level of heat there!
I have found that recent summers are front-loaded in terms of the good weather as the Atlantic ocean is cooler at the start of the season and hence has less energy to fuel weather systems/the jet sream. Later in the summer as the oceans warm the Atlantic has enough energy to barrel through any high pressure that forms over the UK.
I still believe September is a summer month. Autumn only really begins shortly after the equinox when days are shorter than nights (despite this October can be very warm and pleasant, take 2011 as an example)
I'd say August is the best summer month. Summer only really ends on the equinox in my book. Something about the heat and the nights closing in reminds me of the Caribbean where the evenings are very short (i.e. sunset around 7PM)
Temperatures haven't been too extreme here in South Wales as of late. That might be set to change next week if this ensemble forecast for Cardiff is correct. Pleasant temperatures of 30°C with lovely cool nights in the mid to high teens:
Still looking like the first week of August will be the peak in terms of temperature away from the South East. This is the ensemble forecast for Cardiff for example: