This dates back not just to 2015, but all the way to 2009!
I just found this document, from the Chief Nursing Officer and Director General NHS Finance, Performance and Operations dated May 2009, requiring Trusts to implement by 30th June 2009 stating,
" • Trans people should be accommodated according to their presentation: the way they dress, and the name and pronouns that they currently use.
• This may not always accord with the physical sex appearance of the chest or genitalia;
• It does not depend upon their having a gender recognition certificate (GRC) or legal name change;
• It applies to toilet and bathing facilities (except, for instance, that pre-operative trans people should not share open shower facilities);
• Views of family members may not accord with the trans person’s wishes, in which case, the trans person’s view takes priority.
Different genital or breast sex appearance is not a bar to this, since sufficient privacy can usually be ensured through the use of curtains or by accommodation in a single side room adjacent to a gender appropriate ward.
Particular considerations for children and young people Gender variant children and young people should be accorded the same respect for their self defined gender as are trans adults, regardless of their genital sex."
Familiar wording for anyone who has been looking at these!
Full document can be found here: https://assets.publishing.service.gov.uk/media/5a7cd02ced915d6b29fa8d19/CNOnotedh098893.pdf
This document was withdrawn 27th Sept 2019 (though I've yet to locate what is was replaced with) yet was cited as a guiding document for West London's Delivering Same-Sex Accommodation policy.