Johnson, I believe, lives in the flat in number 11.Nor is 10 Downing Street necessarily, it's an office. Tony Blair chose to live in number 11 Downing street as Prime Minister.
.
It is probably fairly reasonable to suggest that the place where someone spends the most nights is usually their primary home. (Obviously have to take into account night shifts, and so on.)
And where someone is registered for council tax, address for NHS, HMRC, etc., purposes.