As I've already said, it doesn't have to be everyone.
People travelling from countries with zero cases of covid-19 may not need testing. People showing absolutely no signs of illness may not need testing.
It needs a degree of common sense and it's not going to be 100% effective. So what if a few people slip through the net and are in the UK sneezing for a few days. This isn't the plague, it's a relatively minor illness for 99% of the people that catch it.
It doesn’t work like that though - the information about where the virus has spread to is days / weeks out of date because of the lag required for symptoms to develop, tests to be done and so on.
Quarantine all flights from China but not (for example) Argentina won’t do much if there aren’t similar controls everywhere - if not then people will still move about passing it on in indirect ways, something already proven by the fact that it appears the majority of our infections have come from Italy.
Also it’s emphatically not the case that this will be minor for 99% of people who get it; depending on underlying health the death rate can be above 10% with severe illness being at least as likely.