I'm guessing that Humberside police has a person who is known in transgender circles and who certain activists call upon to do their dirty work. Could even be Keith Hunter, Police and Crime Commissioner, himself. He seemed to jump in quickly on social media in order to try to minimise this incident.
I'm also speculating that someone more senior at Humberside Police instructed PC Gul to issue this warning to Harry, despite Humberside Police itself having absolutely no remit to do so.
I hope PC Gul has the gumption to realise that he will be the one who is hung out to dry by his superiors (in a rogue officer oversteps his authority story) and moves to protect himself.
This must be investigated.