Brittany Ferries said it had received a “surge” of bookings between the rule changes being announced on Thursday and coming into force on Sunday.
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.