It's not something Johnson is doing. The police and the justice system have been captured by the trans lobby and I doubt if he even knows about it. He's not a details guy.
It wasn't me who said they didn't have GRCs, but the vast majority don't. Actually very few GRCs have been issued compared to the number simply calling themselves trans (and the ones who transitioned before GRCs were introduced). The police and the justice system have been told by the TRAs (Stonewall mainly) that transwomen are women and must be treated as such in every way, and they are going along with it. Even to the point of forcing female witnesses to refer to a defendant they perceive as male as "she".
Newspapers headline "Woman charged with possessing child porn" or whatever, and maybe in paragraph 10 once you've scrolled past the adverts there might be a quick reference to this woman having once been called Bert. It's been written into their code of practice. Again, by transactivist groups (mainly Stonewall) who farm themselves out as consultants and get paid for telling organisations to toe the trans line or else.