People traveling immediately after the holiday may face uncertainty: Many private testing clinics and labs are closed on Christmas Day, so testing within the 72-hour window may prove difficult, especially for the P.C.R. screening, which must be sent to a lab and can take several days to process.
The rapid antigen test, a relatively new tool to detect the virus, gives a result in around 30 minutes, but it is not as widely available, although it is cheaper. Heathrow Airport, for example, charges passengers about $130 for P.C.R. results with 48 hours and about $60 for antigen tests with results within 45 minutes.
Both tests are offered at major British airports — including Heathrow and Gatwick, London’s two major hubs, and Manchester Airport — but passengers must register in advance. It was unclear how many would be able to procure a test and get a result in time for travel.
The introduction of new travel restrictions led to concerns that travelers to the United States would flock to the airport, as Londoners did at train stations last Saturday when tighter domestic rules were announced. But employees at Heathrow on Friday described a normal,