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.