I don't think that the reason had anything to do with Brexit. Besides, I think Johnson hasn't been a similar early denier like Trump was on this issue. If you've followed the discussion here we've talked about the similar policy that Sweden is still following on herd immunity option. They aren't leaving the EU.
On the surface Johnson sounded rational at the beginning, although there is no denying his response was quite low key, for a long time flights were not restricted, passengers weren't checked, the only message was if you have arrived from an infected area, or you have flu like symptoms, make your way home (while interacting freely with the population) and stay there for a week. Followed by a week of saying wash your hands, wash your hands, oh and masks are ineffective.
All quite benign stuff, a balanced moderate approach. But look beneath the surface, the new government which was elected with an 80 seat majority in December literally was the Vote Leave campaign team. The group who hoodwinked the public into voting to leave by claiming there would be hoards of Eastern Europeans and Turks flooding into the country, as Turkey was supposedly about to join. That the migrants already here were the cause of the crisis in our struggling public services, forcing house prices up due to a housing shortage and pushing wages down. The team which masterminded a blizzard of biased and false social media posts, adverts and targeted messages during the last few days before the vote in 2016.
These were the people who had just got their feet around the table in Downing Street as the virus emerged. They were poised to have an almighty row with the EU during the negotiations and make an all out assault on the British constitution to remake it in the image of some Machiavellian fantasy of Dominic Cummings. Cummings had spent 30 years as a radical anti EU campaigner to get to this position. It was the crowning glory of his carrier and just as he reached his goal rumours started spreading about some new virus on the other side of the planet.
It kept getting in the way and the government tried to ignore it to begin with like the spectre at the feast, it warned of global pandemics and social an economic collapse. They had to stay focussed on their modus operandi, to talk the economy(which was struggling) into a good place, to spread the optimism of "make Britain Great again", to negotiate tirelessly and win a great new trade deal with the EU in just a few months. To pull us through a teetering crisis, which the Brexit debacle had become. A Herculean feat.
This was what was going on under the surface when the government adopted its moderate response to the pending outbreak of the virus. Their first concern was to keep the economy on track, so everyone would carry on as normal while we would prepare for heard immunity, which would naturally minimise the impact of the virus in the country. Then the theatened pandemic chaos would go away and they could keep to their Brexit course. If they took hard measures to keep out the virus from the beginning, they risked crashing their whole modus operandi and the economy at the first hurdle. The government would collapse, Brexit would be lost and socialists would get their hands on No10. A total political disaster right at the beginning of Johnson's term in office. He would become a national laughing stock and the privelidged classes would face the wrath of a socialist government.
It was certainly a small price to pay, to lose a few hundred thousand of the countries most vulnerable and old people to preserve all this. Indeed, if all the old people die it would help the Brexit utopia by eliminating the pending social care funding crisis.
Why would'nt Johnson follow the kind of draconian lockdown adopted by countries like China and South Korea?