Air passengers at two of the UK's biggest airports were hit by more disruption caused by a problem with the country's air traffic control system.
It came after Heathrow and Gatwick had already endured a day of flight cancellations and delays due to storms following this week's heatwave.
The UK air traffic controller said the situation would improve after it fixed the problem.
Rail delays caused by extreme heat also began to ease on Friday.
The Met Office said Thursday was potentially the UK's hottest ever day after it received a provisional recording of 38.7C at Cambridge Botanic Garden.
Official figures, which recorded 38.1C in Cambridge, already put Thursday as the hottest July day on record and the UK's second hottest day.
Friday was cooler than Thursday, with temperatures ranging from 23C to 25C in most areas (73F to 77F), rising to 27C in south-east England.
But flights were disrupted by storms across Europe and trains were delayed as rail companies worked to repair the damage from Thursday's extreme heat.
'No idea what to do'
Mark Pickering was due to fly to Singapore on his way to Sydney, but his Thursday evening flight from Heathrow was cancelled due to "spoiled" food caused by the hot weather.
He waited at the airport until 02:00 BST on Friday morning, before getting a hotel and returning at 07:00 to try again.
"I still don't know where my bags are and I've no idea what to do when I finally get to Singapore because I've missed my connecting flight by ages," he said.
"When the flight was cancelled last night there was a line of about 300 to 400 people and no-one really knew what was going on. There was one person at the check in desk."
Diane Styant, who was booked to fly on the same flight to Singapore as Mr Pickering, said she was not able to get any accommodation on Thursday night.
She said she had to sit on an upright chair in a coffee shop until 03:30, when she and her husband joined the queue in departures for rebooking.
After more than four hours, they managed to book another flight leaving on Friday.
"Our precious annual holiday to visit our family who live in Singapore has not got off well," she added.
A "technical problem" at Swanwick air traffic control centre on Friday morning also caused problems at Heathrow and Gatwick.
The UK's air traffic controller said in the afternoon it had "fixed the issue sufficiently to safely increase traffic flow rates".
Heathrow Airport said fewer than 100 flights, or 10% of the total, were affected during Friday.
Both Heathrow and Gatwick advised passengers to check with airlines before travelling.
British Airways said the severe thunderstorms caused "significant delays and cancellations to our operation in and out of London".
At London Liverpool Street station on Friday evening, passengers took to social media to say they faced large crowds and significant delays.
Author Erica James described the scene as "hell", while fellow rail traveller Ross Brereton warned others on Twitter to avoid the station "like the plague".
Network Rail said there were problems with two train lines from the station.
Elsewhere, there was disruption on the Midland Main Line between London St Pancras and Sheffield and on services operated by Great Northern, Thameslink, East Midlands Trains and West Midlands Trains.
West Midlands Railway said overnight work to repair damage caused delays and many trains started on Friday out of place, leading to cancellations and shorter trains.
Passengers using Eurostar services to and from Paris faced "severe disruption" due to overhead power line problems in the French capital.
Many operators said tickets for Friday could be used the following day, and offered compensation to season ticket holders who did not travel on Thursday.
Anthony Smith, chief executive of independent watchdog Transport Focus, said: "All passengers told not to travel due to extreme weather should be entitled to claim compensation."