This is the key case law referenced (given up trying to use italics!):
There are very few cases which deal with this, but I would suggest that this view is supported by the comments of HHJ Jeremy Richardson QC in R (Green) v Secretary of State for Justice [2013] EWHC 3491 (Admin). In that case the Court was dealing with an application by a male prisoner who had murdered his wife and who now wished to be considered female. Green did not have a dysphoria diagnosis but was treated by the prison and the court with the courtesy of being referred to as a woman (presumably socially, given that neither biological nor legal womanhood applied). At paragraphs 66 - 70 HHJ Richardson said that
A comparator has to be found in order for there to be discrimination or for the claimant to show she has had less favourable treatment. The claimant asserts the comparator should be a female prisoner; whereas the governor contends it should be a male prisoner. There can be no doubt the claimant has a protected characteristic gender reassignment. The claimant is, however, male. The only possible comparator is to a male prisoner who is not undergoing gender reassignment.


It seems to me that I must approach the discrimination issues in this way:

(1) Has the claimant been treated less favourably by the Governor than he would treat others in the exercise of his public function?

(2) If he has so treated the claimant, was this due to the claimant's gender reassignment?


Frankly, it is almost beyond argument that the only comparator is a male Category B prisoner at HMP Frankland. I am influenced by the judgment of the Court of Appeal in Croft v Royal Mail Group PLC [2003] EWCA (Civ) 1045. I find it impossible to see how a female prisoner can be regarded as the appropriate comparator. The claimant is a man seeking to become a woman but he is still of the male gender and a male prisoner. He is in a male prison and until there is a Gender Recognition Certificate, he remains male. A woman prisoner cannot conceivably be the comparator as the woman prisoner has (either by birth or election) achieved what the claimant wishes. Male to female transsexuals are not automatically entitled to the same treatment as women until they become women*.


I have no hesitation in saying the correct comparator is a male prisoner in Category B at HMP Frankland. I am utterly unconvinced that the claimant has been treated less favourably than such a prisoner indeed the reverse. Consequently, the second question I posed does not arise.
In other words, where a person has the protected characteristic of gender reassignment but does not have a GRC, their comparator class in looking at a discrimination claim is other members of their birth sex who are not proposing reassignment. Once they do have a GRC, their comparator class is members of their acquired sex.
This means that if the GRA is amended so that self-declaration is all that is needed, Green would have had the comparator class of females rather than males and would have had a good case not just for access to the tights, wigs, prosthetic vagina and sanitary towels sought, but also for removal to the women's estate, because the comparator class would become females. Whilst Green was disqualified for want of a dysphoria diagnosis, the comparator class remained males.
This only applies to discrimination. Clearly, harassment and victimisation (the other two prohibited behaviours under the EA 2010) do not require a comparator class.