Many people assume that the law compels us to use preferred pronouns.
This is not explicitly stated in the Equality Act 2010. Possibly one could interpret failure to use someone's preferred pronouns as harassment, but that seems like a stretch to me (hopefully someone with legal expertise can give proper insight!).
Mermaids and perhaps some police forces want to treat misgendering a crime--to get ahead of the law, as the Labour Party would say.
To quote Susie Green: “We had to get the police involved because a young student was being regularly misgendered by his tutor. The tutor dismissed it until he was informed that it counted as a hate crime.”
www.pinknews.co.uk/2018/02/24/a-charity-called-the-police-on-a-teacher-who-misgendered-their-student/
(Ms Green's legal qualifications aren't known to me. Nor do we have the police's or the teacher's account of this incident.)