Re: Perfect example...
You had a cumulative known total of 321. It was spreading invisibly through the population. This is the only chance you get to stop it.
Other countries with access to exactly the same information and lower case loads saw what was coming took the required action. (NZ, Taiwan, Australia for example).
If you stop the spread early, you can open up early so the total effect on the economy is less. If the UK had taken the required action when it was required you wouldn't have community transmission now and you could open back up. As it is, the virus is out of control and there is no way to safely open up for months.