The Met Office said the temperature of 38.1C in west London was a tenth of a degree higher than the previous record set June 28.
It remained below the UK's August record of 38.5C, set in 2003.
The UK, like France, is experiencing its fifth heat wave of the year, with the first recorded in May.
The latest extreme heat means 2026 has had a record four days with temperatures of 36C or higher.
It also has matched 2022 with two days reaching 38C.
Temperatures in France, meanwhile, have gone even higher, approaching 41C in several areas, from Aquitaine in the southwest to the Rhone River valley in the east, according to the country's weather service.
At least two new grass fires broke out in different areas of the West Midlands in central England, officials said.
Multiple homes were burning in Stourbridge next to a golf course, Sky News helicopter footage showed.
The most intense heat is forecast to move eastward away from the UK and France on Friday and then recede over the weekend.
This northern hemisphere summer has shown that both the UK and France are ill-equipped to deal with these levels of heat.
Air conditioning is a rarity outside of shops and offices, for example.
In the UK, nurses have collapsed from heat exhaustion while working in hospitals, many housed in aging buildings that lack adequate cooling, the Royal College of Nursing said on Thursday.
"When nursing staff are collapsing, feeling dizzy, sick or even being admitted to the very hospitals they work in because their workplaces cannot handle the heat, it shows just how badly they are being failed," said Nicola Ranger, the college's chief executive and general secretary.
Like many parts of Europe, the UK and France are facing widespread drought conditions that have shrivelled crops and drained rivers, heaping pressure on water resources for households, agriculture and the environment, and forcing governments and water users to seek both short- and long-term measures to mitigate the effects of climate change.
Southern Water, which oversees supplies in the south of England, has applied for a rare drought order which would ban businesses from non-essential water use, including cleaning cars and filling non-domestic swimming pools, saying an "exceptional" lack of rain is threatening supplies.
Environment Secretary Angela Eagle will decide on the application, which would be the first such ban since May 2006.
With temperatures soaring, many people headed to beaches and outdoor pools or found other ways to cool off.
In Bordeaux in southwestern France, children and adults sought relief in the city's iconic "water mirror".
Some frolicked in fountains, splashed their faces and filled up water bottles.
Birds too quenched their thirst.
With the land parched and no sign of any meaningful rain coming, authorities in the UK and France remained on high alert for wildfires - while major blazes broke out further south in Europe.
In northern Greece, more than 300 holiday-makers and residents were relocated on Thursday after a wildfire swept through a pine forest and threatened seaside homes, authorities said.
The Fire Service said nine water-dropping planes and seven helicopters were assisting 250 firefighters around the Siviri resort on the popular Halkidiki peninsula.
There were no initial reports of injuries or serious property damage.
In the UK, which is increasingly looking sandy brown rather than its traditional luscious green, firefighters have been out in force this week trying to contain a wildfire in the New Forest in southern England.
A total of 1017 wildfires have been recorded in England and Wales this year, according to data from the National Fire Chiefs Council - the same as last year's record-breaking total.
Prime Minister Andy Burnham has said he will consider imposing a ban on disposable barbecues amid the risk of further wildfires as extreme heat continues to grip parts of the UK.
