Saturday is provisionally the hottest day of the year so far, with 32.7C recorded in Heathrow, the Met Office said.
The UK has now sweltered through six days of temperatures above 30C in September for the first time.
Previously the highest temperature of the year had been set on Thursday, with 32.6C recorded in Wisley, Surrey.
The UK Health Security Agency has issued an amber heat health alert, meaning weather impacts are likely to be felt across the health service, with those aged above 65 or those with pre-existing respiratory or cardiovascular disease at greater risk.
It comes as more storm warnings have been issued across the UK.
A yellow warning issued by the Met Office spans east Wales, Yorkshire, Lincolnshire, Manchester, Nottingham and Oxford is in play until 9pm.
The weather warning means some places could experience flash flooding, lightning strikes, hail or strong winds, with possible interruptions to road access and public transport if such circumstances were to occur.
The same warning has been issued for Sunday covering Northern Ireland, northern parts of England and Wales as well as southern Scotland until 11.59pm.
Environment Agency flood duty manager Chris Wilding warned motorists against driving through floodwaters in the case of flash flooding.
Mr Wilding said: “Significant surface water flooding is possible but not expected across parts of England on Saturday afternoon and evening due to isolated intense downpours.
“We urge people not to drive through floodwater – it is often deeper than it looks and just 30cm of flowing water is enough to float your car.”
Mayor of London Sadiq Khan has issued a “high” air pollution alert for the capital for Saturday, the first since June, and urged Londoners to stop their engines idling and refrain from burning wood or garden waste.
But there is relief from the heat on the horizon, with the UK expected to return to cooler weather next week with a mix of sunshine, showers, and some windy conditions likely as temperatures return towards the average for the time of year.
