I actually got in touch with FPFW asking for clarity on this and Nicola got back to me really quickly!
Basically, according to Section 22 of the GRA2004, it is illegal for any official who finds out that an individual has had the sex on their BC changed via a GRC to reveal this information, except in a very limited number of circumstances.
These are:
(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.
Here’s a link to the relevant section of the GRA2004:
www.legislation.gov.uk/ukpga/2004/7/section/22
While this allows for an individual’s birth sex to be disclosed if, for example, it is directly relevant to a criminal investigation or prosecution, it makes it impossible (illegal) to prove whether a person accessing a sex-segregated space has changed their legal sex.
The “impossible to confirm” part was meant in the context of everyday instances, like access to single-sex spaces and services.
A record of the birth sex is kept but can only be disclosed in very limited circumstances.