JH aren't calculating anything, they are feeding in the official national government numbers. The UK was reporting only pillar 1 tests (hospital labs) up to July 2nd and not pillar 2 (commercial labs - community testing). They changed this and then the official UK sites have now gone back and...
This is actually misleading as cases have continued to fall/stabilise. The uptick you see on the graph is the point which pillar 2 test were added to the daily totals. Official data that now includes that data for all dates - https://coronavirus-staging.data.gov.uk/cases