Who knows? With a government taking this perhaps more seriously, if the response is data-driven and the English understand that this is every bit as serious as the Blitz--tall order, I know--then a shorter time. If a data-driven response & laissez-faire English folk, a while.
If a NON-data-driven response, then the declaration is a waste of time.
@helenS we are already in lockdown! We have been for over a week! Boris said we were wrong to lockdown and wouldnt agree to a joint one! Everything but essential stores are closed here.