Britain is "past the peak" of its coronavirus outbreak, Prime Minister Boris Johnson said Thursday, despite recording another 674 deaths in the last 24 hours, taking the toll to 26,711.

The country is now the third-most affected in the world behind the United States and Italy on cumulative deaths, after changing its reporting to include community as well as hospital deaths on Wednesday.

But Johnson, making his first appearance at a daily government briefing since his own battle with COVID-19, said there were reasons for optimism.

"For the first time, we are past the peak of this disease... and we are on the downward slope," he told reporters.

"We are coming through the peak or rather we are coming over what could have been a vast peak, as though we have been going through some huge Alpine tunnel.

"And we can now see the sunlight and the pastures ahead of us. So it's vital that we don't now lose control and run slap into a second and even bigger mountain."

The government's chief scientific advisor, Patrick Vallance, said the rate of transmission was now below one, with fewer hospital admissions and people in intensive care.

That was having an effect on overall deaths, he said.

"The R (rate of transmission) is below 1. We think it's between 0.6 and 0.9 across the nation. Maybe a little lower in some places, a little higher in others but it's below 1 across the country," he added, referring to the number of people infected by one person with COVID-19.

Johnson's return to work has coincided with increased pressure to lift a lockdown that was imposed in late March to cut close contact transmission of the virus.

A review is due on May 7 and he said a "roadmap" would be published next week about the government's plan to ease restrictions, after concerns about the economic effect of the social distancing measures.

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"What you're going to get next week is really a roadmap, a menu of options, the dates and times of each individual measure will be very much driven by where we are in the epidemic," he said.

"We've got to get your business going again," he added.

"But it's actually vital, if we're to bounce back as strongly as I think we can, that we don't have a second bout of this... because that would really do economic damage."

Experts were looking to develop "ingenious ways" of suppressing the disease that would take into account local conditions, he said, adding that masks could form part of the solution.

"I do think that face coverings will be useful both for epidemiological reasons, but also for giving people confidence that they can go back to work," he said.

Britain saw a sharp spike in its death toll on Wednesday after including out-of-hospital deaths for the first time, though Chief Medical Officer Chris Whitty advised against drawing international comparisons until the pandemic was over.

Johnson spent three days in intensive care with the virus, saying his situation could have gone "either way".

"Let's be frank, tragically, thousands of people have been less fortunate than I was," he said on Thursday in brief comments about his experience.

Vallance said that results of a US trial of the drug remdesivir were a "promising first step", although warned it would not be a "magic bullet" in treating the disease.

Earlier in the day, Britain celebrated the 100th birthday of a World War II veteran whose staggering fundraising efforts have inspired the country in the depths of the outbreak.

"Captain Tom" Moore, who raised more than £30 million ($37 million) for health service charities by doing laps of his garden, was honoured with a Royal Air Force flypast and 140,000 birthday cards.