It's worth noting that in a pandemic situation, waiting before taking action is "making a decision"
Shutting down the economy is costly, yes, but allowing a virus to continue its exponential growth with all of the unknowns that entails is much more costly in expectation (we can only speak in expectation given the unknowns)
Whether or not this model should have been the basis for such a decision I don't know, but it's important to keep in mind that taking proactive steps in a pandemic is conservative
The government wasn't waiting before taking action. The Imperial report was impactful because it said that the government's actions were insufficient, that if they continued on with only moderate social distancing people would die in the streets for lack of hospital beds.
Shutting down the economy is costly, yes, but allowing a virus to continue its exponential growth with all of the unknowns that entails is much more costly in expectation (we can only speak in expectation given the unknowns)
Whether or not this model should have been the basis for such a decision I don't know, but it's important to keep in mind that taking proactive steps in a pandemic is conservative