'the correct comparator for a trans woman without a GRC is a man without the gender reassignment characteristic. If For Women Scotland is correct, that will change if the trans woman has a GRC. The comparator would then be a woman without the gender reassignment characteristic. This would mean that the Equality Act does draw a distinction between those with GRCs and those without them, classing those without a GRC as their biological sex and classing those with a GRC as their legal sex.'
From Foran's article, above.
The EHRC on comparators:
https://www.equalityhumanrights.com/en/advice-and-guidance/gender-reassignment-discrimination
EHRC
'The Equality Act 2010 says that you must not be discriminated against because of gender reassignment.
In the Equality Act, gender reassignment means proposing to undergo, undergoing or having undergone a process to reassign your sex.
To be protected from gender reassignment discrimination, you do not need to have undergone any medical treatment or surgery to change from your birth sex to your preferred gender.
You can be at any stage in the transition process, from proposing to reassign your sex, undergoing a process of reassignment, or having completed it. It does not matter whether or not you have applied for or obtained a Gender Recognition Certificate, which is the document that confirms the change of a person's legal sex.
For example, a person who was born female and decides to spend the rest of their life as a man, and a person who was born male and has been living as a woman for some time and obtained a Gender Recognition Certificate, both have the protected characteristic of gender reassignment. '
So the comparator is a non-trans person.
The comparator is not located in one's sex; it's to do with whether one has proposed or has undertone a process to reassign sex.
But the comparator also has to have enough in common with the person to compare them.
See Citizens Advice's clear advice:
https://www.citizensadvice.org.uk/law-and-courts/discrimination/check-what-type-of-discrimination-youve-experienced/comparators-in-direct-discrimination-cases/
'The comparator is someone who’s in the same or similar enough situation to you, but who doesn’t have the same protected characteristic.
What's meant by someone in a similar enough situation?It’s not necessary for you to be in an identical situation as the comparator. But there must be sufficient similarities between the two of you to show that the reason for the worse treatment is the protected characteristic and not something else.'
A woman who isn't trans has nothing in common with a transwoman. A man who isn't trans has sex in common.