It is not solely his decision. There is a commission of special advisers to do that job. The Mayor of London (the office, not Mr. Khan himself) owns Trafalgar Square, hence this falls under Khan's authority, he being the current elected Mayor.Some would favour a system whereby random busybodies on social media get to choose what goes on the plinth from time to time, but I find the current system more transparent.
Michael Winstanley ● 990d