"serious and imminent threat" to the British

According to the BBC this was because one person in quarantine was threatening to leave, and they did not have the legal power to keep him.

I can't find that article, but a later report (with cases in the UK doubling to 8) has this

BBC wrote: Tackling the coronavirus threat has taken the government into uncharted territory. Quarantining hundreds of British citizens for two weeks has never been done on this scale in modern times. Whitehall sources say the latest Department of Health announcement on the virus threat covers the tightening of some regulations to help enforce quarantine powers. This gives legal underpinning to the quarantining of people back from Wuhan in Milton Keynes and the Wirral. They all signed contracts committing to the 14-day isolation but it's understood that more rigorous regulations are needed to ensure people stay the course.

BBC report

If the brits think that it is a serious threat then it is a serious threat. God help us all. We got more water and a solar set for ourselves. with the cfr being this high and the Ro being at least 3 If not 4, then death is only weeks or months away. For those of us with preexisting conditions and such, this is a threat to our lives. We steel ourselves for this reality as we await the crisis. Prepare the best you can, spare no expense and when it hits, fight as if your existence Is hanging by a thread. The scramble siren has gone off in my head and the metaphorical spitfire or hurricane has been filled with petrol/aviation fuel and the guns have been sighted and rearmed. Let the battle for our lives begin and God speed to you ALL.

The British wouldn't take this lightly. Theres something in 'the wind'. Vigilance is key here. Take sensible precautions. Making a plan is not panic. I think its a consideration for us all to think about in the coming Weeks and Months. If it comes to nothing then thats great news an d we loose NOTHING.

They had to declare it a 'serious and imminent threat' in order to enact the 1984 Public Health Act which gives ministers the ability to introduce emergency powers without the need for legislation, once a “serious and imminent threat” is declared.

This means that people can be legally detained if required.