@raybentos.
The prediction for the number of people who have died is in the scenario where nothing is done to contain the disease. And having examined lots of figures it seems to be exactly accurate.
All the statistics we have show that in the absence of robust public policy aimed at containing the disease, the number of COVID-19 cases in a population multiplies by 4 or 5 per week. This means in particular, given that the average incubation is about 5 days, there are at any given time about three times more people that are infected and will go on to develop symptoms than there are people showing symptoms of the disease. Note that initial studies from Wuhan suggest that a large number of cases go undetected because only causing mild disease.
That means that if we accept the government's own (low case) estimation of 5,000 people in the UK currently infected, if nothing is done then in 4 weeks time there will be of the order of 5000256- 5000625 - ie between 1 million and 3 million infections. Assuming on basis of Wuhan figures a (again, best-case scenario) 10% of patients who need hopitalisation and 2% who need intensive care, this would completely swamp Britain's health care system. Large numbers of people with disease that could in theory have been treated would die because there was not capacity to treat them.
Of course, this is not how things have panned out so far in the countries that have been seriously hit (notably China and Italy- I have no figures on Iran) because in every case, as soon as the health care system started to be in serious difficulty governments introduced robust measures to limit infection. China introduced extreme measures and the epidemic seems to have petered out by now. Within a week or two the measures in Italy (which are not quite as extreme as China's) will have seriously improved things there (yesterday's figures showed, for the first time, no increase in new cases in Lombardy).
The problem is, if you wait for your health service to be almost overwhelmed before acting, you will have no choice but to introduce radical measures, because you need to stop the epidemic straight away before the health care system collapses. Whereas taking strong measures earlier, you have some chance of being able to avoid the radical measures later on.