It really irks me.
No other PM has been referred to in this way (aside from maybe 'Maggie' but even that is usually by a small group and she is mostly known as Thatcher.
We wouldn't call the others Gordon, Tony, John or Theresa. It's either just their last name or both first AND last.
I can only imagine it's because he has managed to carve out an informal 'man of the people' nonsense caricature??