Britons arriving from India must now pay to quarantine in government-approved hotels for 10 days after the country was added to the travel red list.
As of 4am, only British, Irish and third-country nationals with residence rights, including long-term visa holders, can enter the UK.
Other people are banned if they have been in India in the previous 10 days.
The change has been prompted by a worrying rise in infections, with 332,730 cases reported in India on Friday – the world’s highest-ever daily tally which it has now recorded for the second consecutive day – and concerns over a new COVID variant.
The last passengers to land ahead of the restrictions arrived at Heathrow on Thursday evening on a Vistara flight from Delhi.
Among them was a family of three, who were relieved they managed to change their tickets at the last minute, narrowly avoiding having to pay thousands for hotel quarantine.
"It’s really great – we would be having quarantine at home," said the mother – who did not want to share her name.
"That is the best part. And I am having a child with autism so it’s better that he stays at home with us and have all his food preferences at home."
Hotel quarantine costs £1,750 for one adult for 10 days. An extra adult or child over 11 in the same room is £650, and a child aged 5-11 is £325.
University of Stirling student Jetal Zala, who went home to see her parents, couldn’t get a flight to the UK before the restrictions kicked in and will now miss out on campus teaching.
"Our classes were meant to be face-to-face from the month of May," she says. "That’s why I planned to move at the end of April and start studying at campus and have experience of college and university life in the UK."
The travel restrictions are disrupting businesses too.
Ranjit Mathrani, the chairman of MW Eat restaurant group, says most of his highly trained Indian chefs went home during the UK lockdown and are now stuck.
It could jeopardise his plans to reopen.
"Here we are, after five and a half months of lockdown, losing vast amounts of money, looking forward to limping back to normality in the first place, and we are going to be stuck with the probability and possibility of being unable to reopen, except probably on a very restricted basis in some of our key fine dining restaurants," says Mr Mathrani.
The government has said it had to act, with the health secretary saying the decision to put India on the red list was "difficult but vital".
However, some scientists think it should have happened sooner because of the dramatic surge in cases and the new Indian variant, which could have some resistance to vaccines.
More than 130 cases of the "double mutation" – known as B.1.617 – have so far been detected in Britain.
"I think we should have probably seen the writing on the wall, we’ve seen cases increasing in India for several weeks now and it would have been prudent to put India on the red list earlier," says Professor Mark Harris of the School of Molecular and Cellular Biology at the University of Leeds.
"It’s obviously a tough decision because there’s a lot of communication and travel between India and UK, but I think it would have been the right thing to do a few weeks ago."
With India’s cases soaring and many hospitals full, some people have been using hashtags such as #COVIDSOS to share information on medical facilities which may have space – or where oxygen or medicines can be bought.
But hundreds of millions in the country, mainly poorer Indians, do not have smartphone or social media access.
"Access to anything – beds, oxygen, medicines, doctors – is becoming more and more difficult," said children’s rights activist Baidnath Kumar, who has been using WhatsApp to help people in need in Jharkhand state.
"One needs to know someone or appeal on social media for a quick response. But how many people can do that? It doesn’t work."
Indian Prime Minister Narendra Modi has described the situation as a "COVID storm".
So far, India has only administered 130 million vaccine doses out of a population of 1.35 billion.
Currently, only frontline workers and the over-45s are eligible for a jab, but all adults are expected to be allowed a dose from May.
The number of COVID-related deaths in India stands at 186,920 after another 2,263 were reported by the health ministry on Friday.
Total recorded cases are nearly 16.3 million, out of a population of more than 1.3 billion.