There are other considerations, like how well the president/prime minister’s incentives are aligned with the countries. Donald Trump, through very little fault of his own[1], faces the possibility of a failed presidency due to the economic consequences of shutdown. If possible, a president might think it worth swinging for the fences, if he is down a couple of runs in the ninth? If he strikes out, he is no worse than before, although the country is. And Boris has kinda made himself out to be a guy who tries for home runs, at least in his public persona.
There is also the game theoretic considerations on the country level. If all the other countries are doing everything to stop the spread and paying the economic cost, than U.K. can take the benefit of that sacrifice, and not crash it’s own economy.
[1] of course he is doing his best to compound the external factors.