Why the worry though? It was to be expected.
This is why you have modelling. June 21st was based on the Alpha variant which at the time was the dominant strain in the UK. It shows how much we expect cases to increase when things open up and given the predicted vaccination levels it was deemed safe to target June 21st.
This isn't a new thing its happened all along just that its been largely ignored by the government. For example last summer the modelling showed if you opened schools without offsetting it by closing something else cases would rise and we'd end up back in lockdown. Similarly at Christmas, the modelling show allowing a few days of mixing would mean a lockdown in the new year.
Now we've got the Delta variant as the dominant strain in the UK but there isn't certainly over how much more transmissible it is and how it will impact hospitalisations and ultimately deaths so the modelling contains variables. If you look at the Warwick modelling done for SAGE as an example it shows models for 10% increase in transmutability, 20%, 30% etc.
The issue being raised about June 21st is we might not have the data we need to make a more accurate prediction by then. This is because initially the Delta variant had low numbers which means a high degree of uncertainly in any analysis of the data. What we have seen is an exponential rise in cases, even when testing numbers have been lower. Cases started rising last Tuesday. Given the lag between cases and hospitalisations we're only now at the point where we'll potentially see the impact of the rising cases on hospitalisations. And there's an even bigger lag in getting data on hospitalisation by admission date rather than reporting date and vaccination status of those hospitalised.
So far it looks good, hospitalisations and deaths are fairly stable but if in the next few days we start to see a steady rise in hospitalisations we may need to delay the removal of final restrictions to gather more data to ensure its safe. The danger is if we get it wrong and open up too soon as we've seen repeatedly in the past due to the lag by the time we realise its too late we're back to needing a full lockdown which I'm sure everyone is keen to avoid.