You are right @TheOtherPamAyres - they have got the GRA wrong. I was reading up the same documents the other day 
The current Guidance says:
www.app.college.police.uk/app-content/detention-and-custody-2/detainee-care/equality-and-individual-needs/#trans-individual-detainees
Trans-individual detainees
"It is important that trans-individual detainees receive the same respect and dignity as any other members of the public. There is also a requirement to be sensitive to the dignity of police officers called upon to carry out procedures such as strip searches.
Trans-individuals includes a person who has the protected characteristic of gender reassignment (previously referred to as transsexual) and also a person who adopts, fully or partially, dress, appearance and/or behaviour normally identified as belonging to the opposite gender (previously referred to as transvestite).
The term also applies if the person is proposing to undergo, is undergoing or has undergone a process (or part of a process) for the purpose of reassigning the person’s sex by changing physiological or other attributes of that sex. That person is not required to be under medical care to satisfy this definition.
In law, the gender of an individual is their gender as registered at birth unless they have been issued with a Gender Recognition Certificate (GRC) under the Gender Recognition Act 2004 (GRA), in which case the person’s gender is their acquired gender. Annex L to PACE Code C provides useful information on the approach to be followed to establish the gender of the detainee for the purposes of a search or procedure."
So what does it say in "Annex L to PACE Code C" ?
assets.publishing.service.gov.uk/government/uploads/system/uploads/attachment_data/file/729842/pace-code-c-2018.pdf
This is the relevant section of Annex L:
"4. When establishing whether the person concerned should be treated as being male or female for the purposes of these searches and procedures, the following approach which is designed to minimise embarrassment and secure the person’s co-operation should be followed:
(a) The person must not be asked whether they have a GRC (see paragraph 8);"
Paragraph 8:
"(c) Disclosure of information
8. Section 22 of the GRA defines any information relating to a person’s application for a GRC or to a successful applicant’s gender before it became their acquired gender as ‘protected information’. Nothing in this Annex is to be read as authorising or permitting any police officer or any police staff who has acquired such information when acting in their official capacity to disclose that information to any other person in contravention of the GRA. Disclosure includes making a record of ‘protected information’ which is read by others."
Section 22 of the GRA:
www.legislation.gov.uk/ukpga/2004/7/section/22
22 Prohibition on disclosure of information
(1)It is an offence for a person who has acquired protected information in an official capacity to disclose the information to any other person.
(2)“Protected information” means information which relates to a person who has made an application under section 1(1) and which—
(a)concerns that application or any application by the person under section [F14A,] [F24C, 4F,] 5(2) [F3, 5A(2)] or 6(1), or
(b)if the application under section 1(1) is granted, otherwise concerns the person’s gender before it becomes the acquired gender.
(3)A person acquires protected information in an official capacity if the person acquires it—
(a)in connection with the person’s functions as a member of the civil service, a constable or the holder of any other public office or in connection with the functions of a local or public authority or of a voluntary organisation,
(b)as an employer, or prospective employer, of the person to whom the information relates or as a person employed by such an employer or prospective employer, or
(c)in the course of, or otherwise in connection with, the conduct of business or the supply of professional services.
(4)But it is not an offence under this section to disclose protected information relating to a person if—
(a)the information does not enable that person to be identified,
(b)that person has agreed to the disclosure of the information,
(c)the information is protected information by virtue of subsection (2)(b) and the person by whom the disclosure is made does not know or believe that a full gender recognition certificate has been issued,
(d)the disclosure is in accordance with an order of a court or tribunal,
(e)the disclosure is for the purpose of instituting, or otherwise for the purposes of, proceedings before a court or tribunal,
(f)the disclosure is for the purpose of preventing or investigating crime,
(g)the disclosure is made to the Registrar General for England and Wales, the Registrar General for Scotland or the Registrar General for Northern Ireland,
(h)the disclosure is made for the purposes of the social security system or a pension scheme,
(i)the disclosure is in accordance with provision made by an order under subsection (5), or
(j)the disclosure is in accordance with any provision of, or made by virtue of, an enactment other than this section.
(5)The Secretary of State may by order make provision prescribing circumstances in which the disclosure of protected information is not to constitute an offence under this section.
(6)The power conferred by subsection (5) is exercisable by the Scottish Ministers (rather than the Secretary of State) where the provision to be made is within the legislative competence of the Scottish Parliament.
[F4(6A)The power conferred by subsection (5) is exercisable by the Department of Justice in Northern Ireland (rather than the Secretary of State) where the provision to be made could be made by an Act of the Northern Ireland Assembly without the consent of the Secretary of State (see sections 6 to 8 of the Northern Ireland Act 1998).]
(7)An order under subsection (5) may make provision permitting—
(a)disclosure to specified persons or persons of a specified description,
(b)disclosure for specified purposes,
(c)disclosure of specified descriptions of information, or
(d)disclosure by specified persons or persons of a specified description.
(8)A person guilty of an offence under this section is liable on summary conviction to a fine not exceeding level 5 on the standard scale.
(My bolding above).
Section 22 is only about disclosing information once acquired. It does not say anything about the police not being allowed to ask if they have a GRC or to see it.
Section 22 is also clear:
. . . it is not an offence under this section to disclose protected information relating to a person if the disclosure is for the purpose of preventing or investigating crime,