The scientists can forecast and model the likely effects, given various scenarios.
They will know the impact that previous virus' have had, especially a deadly new corona strain, which we have now.
They also said that the NHS would collapse with demand for beds outnumbering supply by 8 to 1.
I can't get my head round Johnson's spinelessness and lack of leadership. If he wants something to happen, like pubs and cafe's closed, he should demand that it's done.