The move to allow larger gatherings outdoors and reopen some sports centers came as many countries in Europe are tightening restrictions as their coronavirus cases shoot up.
LONDON — Across Germany and France, shops are closing, travel is being restricted, and the authorities are ordering people back into their homes. But in Britain, the government moved on Monday to relax its strict national lockdown, allowing people to gather outdoors in groups of up to six people.
The latest cautious steps, announced by Prime Minister Boris Johnson, widened the gulf between how Britain and much of Europe have handled the latest phase of the pandemic. From the rate of vaccinations to new cases of the virus and hospital admissions, the two seem to be moving in opposite directions.
Yet the good news in Britain came with a caveat: Europe’s “third wave” of infections could yet sweep across the English Channel and wash away its hard-won gains. For all of Britain’s success with vaccines, experts warn that without a total ban on travel, no country can be truly immune from another outbreak.
“What we don’t know is exactly how strong our fortifications are,” Mr. Johnson said at a news conference, acknowledging the threat from Europe. “Historically, there’s been a time lag, and then we’ve had a wave ourselves.”
The prime minister spoke from a flag-draped stage in a new briefing room at Downing Street, from which the government plans to conduct regular White House-style press briefings, starting in May. The wood-paneled room was outfitted at a reported cost of 2.6 million pounds, or about $3.5 million.
The setting was apt for what was a delicate exercise in messaging: Mr. Johnson encouraged Britons to take to the outdoors — describing his own excitement at being able to play tennis — while warning them not to assume that the infections, hospitalizations and even deaths from the pandemic are over.
His cautious approach reflects a desire to avoid the mistakes he made last year when he lifted many first lockdown restrictions around the same time. In the government’s eagerness to ignite an economic recovery, it encouraged Britons to return to restaurants by subsidizing millions of meals last summer.
By fall, Britain was suffering a resurgence in cases exacerbated by the emergence of a highly transmissible variant that originated in Kent, in the southeast, and has now circulated widely in Europe and the United States.
This time, Britain’s latest reopening is unfolding in steps — the first of which was a return to schools — followed by several weeks to measure the impact of each relaxation on the spread of the virus. In early April, Mr. Johnson plans to outline his latest thinking on travel and “Covid passports,” a form of certification for those who are inoculated or have recently tested negative.
Further reinforcing Britain’s vaccine rollout, Mr. Johnson announced that the British drug giant GlaxoSmithKline had agreed to manufacture up to 60 million doses of a vaccine developed by Novavax, a biotechnology company based in Gaithersburg, Md., at a factory in northeast England.
Scientists and public health experts generally backed the government’s latest easing, given that it is incremental and encourages mixing outdoors, where the risk of transmission is far lower than in confined spaces.
But they warned about potential vulnerabilities, like the South African variant of the virus, which is fueling the latest wave of infections in Europe and shows signs of resistance to the AstraZeneca vaccine, the one most commonly used in Britain.
“If we didn’t have the variants, I would see us as being in a very strong position,” said Devi Sridhar, head of the global public health program at the University of Edinburgh. “But we have an Achilles’ heel because if a variant develops among vulnerable people, we could be back in a very precarious situation.”
Part of the problem, she said, was Britain’s patchy approach to travel. The government has placed 35 countries on a “red list,” which requires travelers to quarantine in a hotel for 10 days. But it has stopped short of adding France, a high-risk country, because of the headache of dealing with truck drivers transporting freight across the channel.
“Either do all countries, or do no countries,” Dr. Sridhar said. “This selective approach is a little silly because you’re only delaying the problem.”
Though the government could lift the ban on all but essential travel starting on May 17, the third wave of infections in continental Europe has raised the odds that it will either limit or push back its plans.
“They have messed up so often over the last 12 months that they are determined not to be over-optimistic,” said Steven Freudmann, chairman of the Institute of Travel and Tourism, which lobbies for the industry. “There is more political gain in being pessimistic rather than announcing dates and being forced to withdraw them.”
So far, Britain has injected more than 30 million people, or 57 percent of the adult population, with a first dose. Despite some hiccups in supply lines, the government says it is on track to meet its goal of offering a “jab” to everyone over the age of 50 by April 15 and all adults by the end of July.
The rapid rollout, along with the lockdown, has driven down cases and deaths in Britain to a comparatively modest 4,654 cases and 23 deaths on Monday. There were no deaths reported in London for the first time in six months.
With the numbers moving in the right direction, pressure is building on Mr. Johnson for a return to normality.
“Failing to reopen air travel would be devastating to exporters, as well as our tourism and hospitality sectors,” nine Conservative Party lawmakers wrote in a letter to The Times of London published on Monday. “One person’s abundance of caution is another’s redundancy notice.”
Still, in South London, the reopening of open-air swimming pools coincided with temperatures of 65 degrees and brilliant sunshine, leaving clients of the Brockwell Lido pool happy at their limited new freedoms.
“I have been longing to swim and counting down the days,” said Sue Mitchell, a swimming coach who spent an hour with a friend in the pool, where the water temperature was a bracing 48 degrees. “I went a little crazy with excitement waiting to get in — it was like being on holiday.”
Ms. Mitchell said she thought that pools could have reopened earlier but that she supported a cautious timetable for pubs and restaurants to open their doors.
Claudia Dignum, who works for a media agency and who swam 10 lengths with two friends, agreed. “I would rather they don’t rush it, I don’t want another lockdown,” she said after a swim that she described as freezing, but exhilarating, and a step forward after the bleak months of lockdown.