So looks like Mr Johnson senior may have to isolate for two weeks when he gets back. These were the rules when he went to Greece, so he should be prepared for this, but was possibly hoping that as Greece was likely to be exempt, he would also be exempt from isolating.
As it stands, the advice is is clear - "From 10 July 2020, unless they have visited or stopped in any other country or territory in the preceding 14 days, passengers arriving from the following countries and territories will not be required to self-isolate on arrival into England."
He is reported to have stopped in Bulgaria on the way to Greece. Bulgaria is not on the list.
Mind you, he could always extend his stay in Greece to two weeks to avoid the necessity of self isolation.
Regards Margaret