Blistering temperatures look set to evaporate overnight on Tuesday as the heatwave gives way to thundery conditions, prompting yellow weather warnings.
Residents are being urged to be cautious across east and south eastern parts of England, as thunderstorms bring a risk of power shortages and localised flooding.
Commuters can expect bus and rail services to be disrupted, the Met Office said.
Drivers were told to be aware of the risk of spray and a deluge of standing water on roads. The warnings are in place until the early hours of Wednesday.
It comes as the mercury plummets by seven degrees in some places, as highs of 30C give way to 23C in London.
The forecast for Wednesday will see sunshine and showers across northern and western parts, with heavy rainfall in places.
There will be “dry with sunny spells across the south and east, but much fresher than on Tuesday,” the Met Office added.
Temperatures across the UK will be milder than those Britons have grown used to. Dundee, Scotland, will see a high of 18C on Wednesday, while Swansea, Wales will see a high of 19C. Coleraine, Northern Ireland, will reach highs of 17C.
The outlook continues for the rest of the week, with Saturday currently predicted to be a washout.
It comes as the Office for National Statistics said it was too early to tell if the recent heatwave had led to an increase in early deaths.