See Equality Act 2010, Schedule 3, section 27 [single sex services]; subsection 6:
(6) The condition is that—
(a) the service is provided for, or is likely to be used by, two or more persons at the same time, and
(b) the circumstances are such that a person of one sex might reasonably object to the presence of a person of the opposite sex.
www.legislation.gov.uk/ukpga/2010/15/schedule/3/part/7/crossheading/singlesex-services
This legal provision provides the legal right for any female to object to any person perceived to be of the opposite sex in their single sex space [eg: female only communal toilets, that is toilets used by more than one female at the same time ].
So, we females always have the legal right to have excluded from our single sex spaces anyone who is male. The possession of a GRC does not supersede this basic legal right.
See Equality Act 2010, Schedule 3, section 27 [single sex services] and section 28 [gender reassignment]:
www.legislation.gov.uk/ukpga/2010/15/schedule/3