The quarantine, which came into effect at 4am, applies to those coming from France, the Netherlands, Malta, Monaco, the Turks & Caicos Islands, and Aruba.
Tens of thousands of UK tourists in France made last-ditch bids overnight to return home before the imposition of quarantine restrictions.
Tickets for planes, trains and ferries were snapped up by travellers at increased prices as they attempted to beat the deadline.
The dash to get home was prompted by the decision to impose the quarantine on travellers from France due to rising numbers of coronavirus cases in the country.
UK transport secretary Grant Shapps insisted the British government had taken “a practical approach” to the new restrictions.
The quarantine conditions also apply to travellers returning to or visiting the UK from the Netherlands, Monaco, Malta, Turks & Caicos and Aruba.
Eurotunnel Le Shuttle, the train service which carries vehicles through the Channel Tunnel, was fully booked on Friday.
A spokesman said 12,000 people tried to book tickets in the hour after the new rules were announced at about 10pm on Thursday, compared with just hundreds normally.
Some air fares were more than six times more expensive than normal.
British Airways was selling tickets for a flight from Paris to London Heathrow on Friday night costing £452 (€500).
The same journey on Saturday could be made with the airline for just £66.
The cheapest ticket on a Eurostar train from Paris to London was £210, compared with £165 on Saturday.
Travellers in the south of France and the Netherlands faced a struggle getting back to the UK in time as many direct fights were sold out.
Holidaymakers Stuart and Anna Buntine spent nearly £1,000 to make it home via Eurostar from Burgundy, central France.
Mr Buntine (58) said: “I went to bed last night thinking it was all OK, woke up at 7am to find we had to get back here pretty sharpish.
“We couldn’t get tickets, all the sites had crashed… we had to buy business class tickets back today so it’s cost nearly £1,000.”
Mrs Buntine added: “We left here with our eyes (open) knowing that it was a possibility, so we decided we’d take that risk.”
Dyan Crowther, chief executive of the HS1 high-speed London to Channel Tunnel rail link, told PA it was “heartbreaking” seeing families having to cancel holiday plans and spend hundreds of pounds dashing home to beat quarantine.
She said: “People want certainty, they want to know that they can go away without having to worry about what the world will look like when they return.
“My heart goes out to them.”
A spokeswoman for British travel trade organisation Abta said the UK government’s quarantine policy will “result in livelihoods being lost unless it can step in with tailored support for the travel industry”.
The UK's Joint Biosecurity Centre and Public Health England reported a significant change in Covid-19 risk in all six destinations for which quarantine restrictions are being imposed.
The UK Home Office said on Friday that a total of nine fines have been issued at the border since quarantine restrictions were introduced.
The department counts the number of fixed penalty notices issued by the UK Border Force under the regulations.
