Sadly at the moment down here among the God forsaken heathen an open window does nothing more than allow a hot sweaty soup of muggy air to flow in.
That's why you open them at night, then close them during the day and let the house insulation keep the heat out. There shouldn't be any need for air con in the Midlands yet this year. Even my retrofitted 35-year-old house can keep cool without it. I thought yours was newer.
A few years ago, where we had a period of sustained 40ish days and 20ish nights, I couldn't cool the house sufficiently overnight and so lost control of the temperature after three days. Then air con might have been useful. For about a week. Still not worth it.
Of course, cars need air con more often, being basically mobile greenhouses, but that can be reduced by using bikes for short journeys. Amusingly, even little "urban heat islands" are better to cycle around because English planners tend to shove cycleways around the edges, where you're in the flow of cool air getting sucked in to go up in the thermal currents generated by the heat island, often with boundary trees offering some shade... meanwhile, motorists are sat in the middle of eight lanes of sweltering blacktop far from any shade... and it's really dark blacktop, without the lighter stone or concrete mixed into road surfaces in more Mediterranean countries, so it gets really hot. Scorchio!
Edit to add: read more. Oh yeah. MS. That makes air con worthwhile. Still shouldn't really be needed yet. Also saw your post reminding me about the extreme solar gain of your windows. I'd add shutters, you added film. Now the test!
