I see lots of folks round our way with the windows open on hot days. I've been keeping all the windows closed and blinds down during the day, open it all up in the evening and morning to get the heat right down for the next day, works like a charm. If it does ramp up from this over the next few decades, we're definitely going to have to put the infrastructure in place for the AC load and decent insulation.
Interestingly enough, solar panels work much better when it's cold. There are a lot of improvements the UK could make to mitigate this problem for winter and summer. Solar panels are a must, followed by good insulation and subsidies for heat pumps ac pumps.