First, calling a trans woman a man is misgendering, which is transphobic even if unintentionally.
Secondly, it is not accurate and precise enough. A trans woman is not a man, but she is not just any woman, either. Accuracy and precision are important for good record keeping, for research purposes, and for correctly informing the public.
Thus, a transgender rapist should be recorded as such in police and cps records, and should be referred to in press releases as per Stonewall guidelines (transgender woman, then trans woman for subsequent mentions).
If there is any doubt, I suggest a default of assuming a suspect is trans, to avoid giving offence. Stonewall guidelines make it clear that trans is an umbrella covering many things, including men who only very occasionally cross-dress, for whatever purpose.
It would help if all press releases included an explainer box about what a trans woman is, given the surprising number of people who don't seem to know.
This approach may make the use of 'cisgender' (eg for other suspects) unavoidable, which I know some will dislike. But avoiding ambiguity about what is really happening seems more of a priority at the moment.