I don't know what part of the Act Appendix L is in but there are several statutory Codes. POLICE AND CRIMINAL EVIDENCE ACT 1984
CODE A
CODE OF PRACTICE FOR THE EXERCISE BY:
POLICE OFFICERS OF STATUTORY POWERS OF STOP
AND SEARCH
POLICE OFFICERS AND POLICE STAFF OF
REQUIREMENTS TO RECORD PUBLIC ENCOUNTERS
This has a section about recording sex, or as the heading has it, Gender:
ANNEX F ESTABLISHING GENDER OF PERSONS FOR THE PURPOSE OF
SEARCHING
Paragraph 3 uses sex and gender:
[In part]:
"Consideration
3. At law, the gender of an individual is their gender as registered at birth unless they possess a gender recognition certificate as issued under section 9 of the Gender Recognition Act 2004, in which case the person’s gender is the acquired gender.
(a) If there is no doubt as to the sex of a person, or there is no reason to suspect
that the person is not the sex that they appear to be, they should be dealt with
as that sex.
(b) A person who possesses a gender recognition certificate must be treated as
their acquired gender.
(c) If the police are not satisfied that the person possesses a gender recognition
certificate and there is doubt as to a person’s gender, the person should be
asked what gender they consider themselves to be. If the person expresses a
preference to be dealt with as a particular gender, they should be asked to sign
the search record, the officer’s notebook or, if applicable, their custody record,
to indicate and confirm their preference. If appropriate, the person should be
treated as being that gender."
[Continues]
NOTE: This version says copyright 2010. They are updated when there's new legislation that PACE applies to or for other changes.
https://assets.publishing.service.gov.uk/media/5a7a3076ed915d6eaf154650/pace-code-a-2011.pdf
I expect the College of Policing is somewhere in the mix.