Fully vaccinated people can now visit France without quarantining on their return to the UK, after travel rule changes came into force at 04:00 BST.
But travel agents said the relaxation of quarantine for one of the UK's most popular destinations came "too late to save the summer".
Meanwhile, travellers in Mexico had to rush back to avoid hotel quarantine.
Under the latest changes to the UK's traffic light system for travel, the rules have been relaxed for a dozen countries.
France moved from its own amber-plus category onto the amber list, which means children and fully vaccinated passengers do not have to quarantine on their return, although unvaccinated travellers still have to self-isolate at home for 10 days.
The UK government sets the red, amber and green lists for England, while the other nations are in charge of their own lists. Scotland, Wales and Northern Ireland confirmed they will be adopting the same changes as England - although Wales criticised the UK government for not consulting them.
And India, Bahrain, Qatar and the United Arab Emirates have been moved from the red list to the amber list.
Countries have their own rules about allowing visitors - so being on the UK's green list does not guarantee travellers can visit.
Four destinations are also subject to tighter restrictions. Mexico, Georgia, and the French overseas territories of La Reunion and Mayotte in the Indian Ocean, have been added the red list.
Only British citizens, Irish citizens, and UK residents can travel from red list countries and they must stay for 10 days in hotel quarantine at a cost of £1,750 for one adult.