Assuming there is no vaccine, to know when to exit lockdown we need accurate figures on how many people have the virus and are infectious, how many people have had the virus and are now immune and how many have not had the virus. If enough people have had the virus and are immune then they pose no threat to anybody and lockdown could end. Ironically, England may be suffering most now but equally well it may have many more immune people then the other constituante countries and reach release sooner.
The major challenge is getting the accurate figures needed. As the ONS report has made clear today.