There are 13 year old guidelines on this from ACPO. It is not complicated to find this out.
www.lgbthistorymonth.org.uk/documents/ACPOGuideJul05.pdf
"If there is any question over the detained person’s gender you should ask the individual in which gender they wish to be addressed. A person who is 'en femme' may be comfortable with their male birth gender and wish to be treated as such. They may want to be addressed and dealt with as a female as that is the gender they identify with. Never assume sex or gender identity, each trans person will be unique in how they want to be treated. If asked sensitively you will not offend and you can simplify the process."
It is total nonsense that the police are concerned with 'legal gender'.
TRAs lobby for this stuff, they get what they want. And they've had it for the last 13 years.
Note that this all comes from the European Convention of Human Rights - more specifically how it is interpreted by modern judges. The 'right to privacy' is now taken to mean that it's an impossibly offensive abuse of human rights to check on these things. And of course it's all centred on the individual's rights, not those of the possibly hypothetical people they might come into contact with.
Within the context of platitudinous general human rights principles and modern TRA lobbying, much of what people on here imagine to be important simply isn't.
Is it important to identify a male rapist as a man - NO, it's seen as much more important not to brutally violate his human rights by describing him as a man.
That's just how human rights legislation works.
There's not really a solution to that, other than changing liberal thought, which certainly hasn't happened. (In less liberal past ages, the exact same legislation would NEVER have been interpreted to mean a man can declare himself a woman, and nor would that necessarily be the case in countries with more conservative judiciary.)