I don't want to derail the thread but, as an example, the Girls' Day School Trust has been very clear on this - and predictably has had loads of criticism e.g. from Penis News.
Excerpts from GDST Gender Identity policy:
The GDST is committed to single-sex education for girls. Admissions to GDST schools are based on the prospective student’s legal sex as recorded on their birth certificate.
GDST schools are able to operate a single-sex admissions policy, without breaching the Equality Act 2010 on the basis of an exemption relating to biological sex. The GDST believes that an admissions policy based on gender identity rather than the legal sex recorded on a student’s birth certificate would jeopardise the status of GDST schools as single-sex schools under the act. For this reason, GDST schools do not accept applications from students who are legally male.