"Britain's new midsummer heatwaves are [..] likely to have severe consequences. European law states that governments must inform the public when hourly concentrations of ozone rise above 180 microgrammes per cubic metre. On 6 August last year, ozone levels over London peaked at 300 microgrammes. Other high spots were found in East Anglia and the Midlands."

I remember reading this article two years ago during the record-breaking heatwave. It was discovered afterwards that many of the deaths were caused not by heat stress and dehydration but by ozone. I haven't heard anyone mention this phenomenon this year though...