I think - unless I'm mistaken - Marcus Rashford's campaign was a national campaign.
Yes, I wish Khan was PM (it would cause social media to implode with wingnuts) - but he's not even the LOTO.
Prime Minister - Boris Johnson - responsible for U.K.- wide legislation.
Parliament - provides oversight. At present there's an 80-seat Conservative majority, so Johnson basically has rubber-stamp government.
Leader of the Opposition - Keir Starmer, Labour leader. Presently facing an 80-seat majority, so has limited power.
Mayor of London - Sadiq Khan. Limited powers, in London. Range of powers extremely limited by successive Conservstive governments. Previous Conservative Mayor Boris Johnson) partly a stooge, who didn't challenge the limited nature of Mayor's powers. Chiefly responsible for spaffing vast amounts of money and being workshy.