The July record for the highest overnight minimum temperature is 23.3C - and the Met Office says that could be surpassed in south-east England on Thursday.
Rail services across the United Kingdom were also disrupted by lightning strikes.
The English end of the Folkestone-Calais link was worst affected, with people forced to queue for up to 90 minutes just to check in and a further three-and-a-half hours for the next available slot, according to the operator.
Experts at the Met Office have issues a Level 3 heatwave warning for parts of the United Kingdom - just one step away from declaring a national emergency.
The Society of Acute Medicine has also raised concerns about patients and staff in overheated hospitals. "Also try to carry water with you when travelling".
Temperatures were around 21C in parts of East Anglia and London past midnight.
The all-time British record is 38.5C set in August 2003 near Faversham in Kent, southeast England, while the highest July temperature was 36.7C in 2015 at Heathrow airport.
UK weather forecast The thunderstorms will replace the heatwave we have been experiencing
The Met Office has has issued a severe weather warning for thunderstorms across eastern Britain from 2pm today and again tomorrow afternoon. Maximum temperature will be 20C.
Temperatures had cooled over the last few days but the warm weather is due to come back this week.
Spray and sudden flooding could also lead to hard driving conditions and some road closures.
However, forecasters predict that the hot weather will continue next week after the stormy weekend.
There is the potential for thunderstorms to develop during this afternoon, lasting well into the evening and perhaps developing into a larger area of rain.
Where these thunderstorms do take place, torrential downpours with a much as 30 mm of rainfall in 1 hour and 60 mm in 3 hours could occur.
The heatwave baking Britain comes as extreme hot weather grips northern Europe, with scientists warning that the kind of conditions being seen at the moment in much of the northern hemisphere are made more likely by cliamte change.
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