Here we have a newly mutated virus to which the human immune system doesn't have a defence yet. The virus spreads rather easily and is moderately nasty, killing between 1% and 7% of those who catch it after about 14 days. There is no cure, but several effective palliatives. It appears most dangerous to elderly persons who already have a serious health condition.
I've only seen figures for China where it took 20 days for the number infected to rise from about 500 to about 40000. If I've done the sums right, the number of patients infected doubles every six days or so. But, the spread in China and other countries has been slowed by aggressive containment measures – it hasn't accelerated into the population anywhere yet.
Left unmanaged a virus will spread freely until it can't find any fresh hosts. Although everyone risks catching it, not everyone will get it, and not everyone will spread it. People who have recovered cease to be fresh hosts and tend to stop the spread. So the epidemic will peak and subside, perhaps taking 2 or 3 months. (Modelling the disease is a job for experts!)
Worst case, 80% of the population will get it and – if UK mortality reached Italian levels – there would about 2.9 million deaths here. More realistically, 40% of the population will get it and mortality will be only be 1%, resulting in about 240,000 deaths. (UK normal mortality: 56000 people died here in the month January 2020.)
Hand-washing reduces the likelihood of spreading but isn't as effective as isolating all those who have been in contact with an infectious victim. Testing is vital because it allows new infectious contacts to avoid others, thus snubbing viral progress. However, it is all but impossible to stop a viral epidemic because there are so many different ways it can spread – we have to eat, see doctors, relatives and work etc. It will spread, and 40% to 50% of us catching it isn't unlikely.
Government tactics appear to be aimed at extending the time it takes the virus to achieve full penetration rather than ensuring everyone will be fully treated on demand. If the virus is allowed free rein the sheer number of severe cases arriving in a short space of time is likely to overwhelm the Health Service. That's like the flood defences failing. However, if the epidemic is spread over several months, the Health Service should be able to cope.
No need to panic, but I think we will have to tough it out somewhat.
Dave