I agree with the question why is Boris being asked to self isolate but not the reasoning.

Having had to self isolate myself from T&T the definitions for having close contact given to me were either
1. Face to face contact with someone who has been tested positive for covid to a distance of one metre even if wearing a mask
2. Or being within 2 metres of an infected person for 15 minutes or more

Why is anyone, except for family (or your lover) putting themselves in either position, especially someone in his position, social distancing? It also explains why so much of the spread is via family and friends.