I think all this "policy guided by the science" is a bit misleading. Yes, the science - in this case modelling of the likely impact of the virus and the effectiveness of individual measures on its spread - provides some basis for decision making. However, this knowledge is partial, subject to considerable uncertainty and rapidly developing.
The civil service will take what information is available to them at the time - which will be a fairly small subset of the total information available. They will then work up possibly practical actions that could be taken, setting
out possible pros and cons. It is then up to the politicians to decide what happens - at least partly influenced by their political outlook, gut feelings and fear of seeming out-of-step with decisions taken by other governments. After all, who wants to be the brave one here and go against the flow?
I am sure all our leaders are trying their best to make the right decisions, the science is very important in this - but it is only one element.