Scientists, though, warned that thousands of new ventilators may come too late while the government said it failed to join a European scheme to boost capacity because of a communications “mix up".
Britain initially adopted a light-touch approach to the outbreak but has since imposed tougher measures, including a three-week lockdown, as confirmed cases and deaths climbed.
As of 1700 GMT on Wednesday, there were 11,658 confirmed cases of COVID-19 and deaths surged past 100 in 24 hours for the first time to bring the total to 578.
The chief executive of NHS Providers, which represents the heads of hospital trusts in the state-run National Health Service, said there had been an “explosion of demand" in the capital.
Chris Hopson told BBC radio that hospital bosses said there had been “wave after wave after wave" of admissions of seriously ill patients, with a surge in numbers predicted in the coming weeks.
“The word that's often used to me is a sort of continuous tsunami," he added.
Official statistics are thought to represent just a fraction of the real number of infections across Britain, as only those taken to hospital with severe symptoms of COVID-19 are tested.
Johnson on Monday imposed a shut-down of non-essential shops and services, and banned gatherings of more than two people to cut close-contact transmission of the virus.
In a sign of an expected rise in cases, 500 beds will be available from next week in a new 4,000-bed field hospital being set up at the giant ExCeL exhibition centre in east London.
But despite urging people to stay at home, London Underground trains and platforms have been packed with people unable to work remotely, particularly non-contract construction workers.
Britain has unveiled a raft of unprecedented government support to help businesses and employees as the economy slumps, and on Thursday extended it to the self-employed.
Finance minister Rishi Sunak unveiled a scheme offering grants worth 80 percent of a self-employed person's average monthly earnings in the last three years up to £2,500 ($3,000, 2,750 euros) a month.
The scheme would last at least three months, with payments backdated from March 1, he told a news conference.
“What we are doing will, I believe, be one of the most significant economic interventions in the history of the British state and by any government anywhere in the world," he said.
Frontline healthcare workers say a lack of personal protective equipment (PPE) as well as insufficient testing of staff for COVID-19 is putting them and patients at risk.
“If hospitals are to survive this, we urgently need adequate protective clothing. Otherwise we are lambs to the slaughter," one doctor wrote in the Daily Mail newspaper.
The government says it has shipped 7.5 million pieces of PPE in the last 24 hours but Hopson said “unprecedented" staff absences of up to 50 percent were exacerbating the crisis in London.
The NHS has access to some 8,000 ventilators and the government has ordered 8,000 more. Johnson's spokesman said they were expected to arrive in the coming weeks and months.
But Imperial College epidemiology professor Neil Ferguson, a government scientific advisor, has said most would be needed “in approximately two to three weeks" time as demand peaked.
Entrepreneur James Dyson said the government had ordered 10,000 ventilators from his firm but Downing Street said that depended on the machines passing safety and regulatory checks.
Opposition political parties meanwhile blamed Brexit for Britain not taking part in a European Union scheme to procure more ventilators.
But Johnson's spokesman said it was a communication “mix-up" which meant they missed the deadline.
“We will consider participating in future procurement schemes on the basis of public health requirements at the time," he added.
