The alert system chart was not very well done in the first place, and it was crafted with the situation of that particular moment in mind. A period that was all about the lack of plans for dealing with the relaxation of measures post first wave peak.
This problem is especially prominent in the action column for level 4. The wording 'Current social distancing measures and restrictions' is wording of its time, reflecting the measures that were in place when the chart was written, at a time when they were looking forward to us gradually moving down the levels, not going back in the other direction.
Otherwise, if the chart wording is taken literally, restrictions arent reintroduced until level 5, even if 'current' restrictions up till that moment were very weak. And one of the big lessons about a level 5 situation is that you have to act before you get to that situation, otherwise only the strongest full lockdown measures are left as a viable response.
Another issue is how to tell exactly when its appropriate to say we are at level 5 rather than level 4. Because the description of level 5 makes no mention of how large, imminent and unavoidable the 'risk of healthcare services being overwhelmed' needs to be. For example it is not necessary to wait till healthcare services are about to be overwhelmed before judging that this risk is very real. And that matters right about now since the likes of Whitty will already have been able to warn the government that the current estimate for R means the current trajectory will lead to the NHS being overwhelmed later this year.