Airports are expected to be busy over the weekend after massive outages hit computer systems globally.
Journeys could still be affected by delays and cancellations, and a number of airports have said passengers should continue to check for potential disruption before they travel.
The outages came on one of the busiest days of the year, with many schools in England and Wales having broken up for summer this week.
GPs, pharmacies, banks, payment systems, and train services were also affected.
The problems were caused by a faulty software update for Microsoft Windows issued by cyber-security firm CrowdStrike.
Speaking to BBC Radio 4’s Today programme, computer scientist Sir Nigel Shadbolt said most IT systems similar to CrowdStrike’s software work to “very high levels of quality”.
But “it’s like a pandemic” when they go wrong, he added, saying lessons should be drawn.
Mr Shadbolt said using multiple systems may be the way forward.
In a blog post on Saturday, CrowdStrike CEO George Kurtz apologised and said his company “understands the gravity and impact of the situation”.
London Heathrow was operating as usual this morning but with added footfall, as passengers who were due to fly on Friday continue their journeys over the weekend.
London Gatwick said its system were “operating as normal” but that “some delays and cancellations will however continue” in the next couple of days.
Manchester Airport, where check-in was for a period on Friday being carried out manually, said its check-in systems were up and running but that flights “may still be subject to delays and cancellations”.
Belfast International said it was operating as normal but that “we recommend passengers keep in contact with their airlines”.
The Port of Dover said it was not experiencing IT issues on Saturday morning, but but hundreds of people unable to fly on Friday are coming to the port, some without ferry bookings.
There is a two-and-a-half hour wait at the port, ferry firm DFDS said.
As of Friday evening, 338 flights in and out of the UK – 167 departures and 171 arrivals – had been cancelled, according to aviation analytics firm Cirium.
Simon Calder, travel correspondent for the Independent, said the passengers he had spoken to at airports were “absolutely despondent”.
Speaking to BBC Breakfast on Saturday, he said the ordeal was “horrible” for those hoping to travel in and out of the UK.
“There are 50,000 British travellers waking up this morning where they didn’t expect to be,” he added.
Delayed and cancelled flights can often have knock-on effects beyond the initial disruption because planes and crews are not where they had been scheduled to be.
On the rail network, National Rail said the issue was no longer affecting operators, though some operators are still advising customers to check their journey before travelling.
The outages meant several retailers – including Morrisons and Waitrose – were unable to take card payments for the part of the day, while TV channel Sky News was briefly off air.
GPs struggled to access their records systems, which include online bookings. Pharmacy services, such as access to prescriptions, have also been affected.
On Friday, Mr Kurtz said he was “deeply sorry for the inconvenience and disruption” caused by the outages, though stressed that they were not the result of a security breach or cyber attack.
He later told the company’s customers in an open letter: “You have my commitment to provide full transparency on how this occurred and steps we’re taking to prevent anything like this from happening again.”
