It's because you don't know how to manage it. For a short term, few hot days, the trick is to air all night and shut everything up early in the morning as soon as the temperature starts rising. Your house will trap cold just as well as warm. But British people think that open windows = colder so they mess this up every time. I lived in the UK for the last three summers, in a perfectly ordinary row house from the 70s. It was fine with normal heat management strategies that one would use in eg Germany. Yes, if a heatwave lasts a long time this strategy will start to fail, if you can't cool properly at night, but it works for at least three days of over 30°.