Boris Johnson probably is the most popular Conservative in the UK, although I acknowledge this is not saying much. Nevertheless he strikes a chord with the electorate that Cameron and Osborne can only dream of.
This week he has been decidedly strident in his comments about EU membership, the move towards greater political and fiscal union within the eurozone and plans circulating in the EU for an EU banking supervisor. Wearing his Mayor of London hat he is is justified in commentating on issues which might affect London as a financial centre and this inevitably spills over into other European issues. However, he gives the impression of distancing himself from the Downing Street Two and giving tacit support to Tory rebels on EU issues.
A Leader of the Conservative Party in waiting? He would have to be an MP before making a leadership bid, so it may be that he is waiting for the result of the next general election before making a move. In any event I believe he is on record as stating he will see through to completion his current term as Mayor of London.