The ECB made exceptions when they've wanted - see The Hundred carrying on despite cases in support staff and players. Whereas other domestic cricket has had many matches cancelled.England is a country which has eased almost all the restrictions after their double vaccination thing.
With no strict bubble a case up here & there is bound to happen and shouldn't be a surprise at any cost since COVID cases will likely happen even after double vaccination.
So either you maintain your bubble thing or be prepared so such outcomes.
And currently it seems like ECB was not expecting the latter to happen.
I'd be surprised if the game doesn't go ahead. Although I expect it will depend on the outcome of today's testing.