When London-based cardiologist Dr Mark Gallagher left work at a coronavirus-hit hospital just over a fortnight ago with a 37.9 degrees Celsius fever, he feared the worst.

Dr Gallagher had a strong view he had been infected, and a swab test later confirmed he had COVID-19.

"My signs were clear cut, absolutely sudden and severe," he said.

"So I knew what it was but everyone needs their own thermometer, everyone needs to be checking their own temperature because the signs can be subtle."

Having just returned to work after spending time in isolation, Dr Gallagher says the UK government has been too slow to react to the danger signs of the outbreak.

"Of course, they [the government] were fully alerted to the problem in the first week of January, so the response is extremely disappointing," Dr Gallagher told the ABC via videocall.

"It is very amateur."

It comes as the country recorded its largest daily death toll of 563 on Wednesday (local time), taking the total number of deaths to 2,352.

Last week, Prime Minister Boris Johnson ordered a virtual lockdown of the world's fifth-largest economy to prevent the spread of the COVID-19 virus, banning Britons from leaving their homes for all non-essential reasons.

People are only allowed to leave the house for one of four reasons — buying basic necessities, exercising once a day, medical needs, or travelling to and from work.

Lesson has to be 'test, test, test, identify and isolate'

Britain's National Health System is under strain from the high number of COVID-19 cases in the UK. ( Reuters: Hannah McKay )

While stricter measures have now been implemented, Dr Gallagher was critical of the Government's initial discussion around a herd-immunity strategy, when studies showed it could lead to the death of between 200,000 and 500,000 people.

"That delay strategy, aiming for herd immunity, to my mind that was an attempt at a crime against humanity," he said.

After the UK's chief medical officer discussed herd immunity as a strategy to combat coronavirus, the UK Health Secretary Matt Hancock insisted it was not Government policy.

"Herd immunity is not our goal or policy, it's a scientific concept. Our policy is to protect lives and to beat this virus," Mr Hancock said.

Now what is of particular concern for many medics is the slow rate of testing for COVID-19.

Last week, the UK government bought 3.5 million antibody coronavirus test kits, to be available for purchase from pharmacies and online, which it said will allow people to test themselves for the disease at home.

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The latest daily figures show over 7,000 people were tested for the virus within a 24-hour period and of those 4,324 returned a positive result.

But Mr Johnson, who himself remains in isolation with COVID-19, had initially set a target of 25,000 tests a day.

Space to play or pause, M to mute, left and right arrows to seek, up and down arrows for volume. Watch Duration: 2 minutes 12 seconds 2 m 12 s UK Prime Minister Boris Johnson says he has developed mild symptoms of coronavirus

Health officials now admit that figure won't be reached until at least the middle of this month. That's despite Germany testing about 500,000 people per week.

"So the lesson has to be test, test, test, identify and isolate," Dr Gallagher said.

Mr Johnson also addressed the issue of more testing in a Twitter video posted on Wednesday.

"We're also massively increasing testing," he said.

"And I want to say a special word about testing because it is so important and as I have said for weeks and weeks, this is the way through, this is how we will unlock the coronavirus puzzle and how we will defeat it in the end."

Mr Johnson said what the country needed to do was massively ramp up tests to help give clarity to people who do and don't have the virus.

This is how the doctor coped during self-isolation

The ABC began speaking to the surgeon during his isolation in his London home a week ago.

He was using a spare room in the attic with an en suite to stay away from his own family, and even sealed the door with wedges of paper and clothes to completely stop the flow of air.

"Anyone can do this for a few weeks, but I'm very fortunate that we have a suitable room," he said.

"It's the en suite that makes this possible."

A bang on the wall signals the arrival of a meal that is left outside, before Dr Gallagher puts on a surgical mask and opens the door briefly to collect the food that is placed on a disposable plate.

"It's okay here for me, I don't ever let anything out of the room," he said when in isolation.

"But in most homes that is not going to be possible.

"This is one of the lessons from Wuhan — one of the very disappointing things about the world response to this is the failure to learn lessons from China."