Section 7(1) equality act 2010 refers to “physiological or other aspects of sex.”
What are these other aspects of sex? In other words, what does this “or other” clause refer to?
The cited provision is:
A person has the protected characteristic of gender reassignment if the person is proposing to undergo, is undergoing or has undergone a process (or part of a process) for the purpose of reassigning the person's sex by changing physiological or other attributes of sex.
This language originates in the Sex Discrimination (Gender Reassignment) Regulations 1999. This statutory instrument was made in response to a 1996 decision of the European Court of Justice (P v S and Cornwall County Council, C-13/94) that Council Directive 76/207/EEC of 9 February 1976 , regarding equal treatment of men and women, also covered discrimination on the grounds of gender reassignment. The regulations were made in order to bring domestic law up to date with the EU requirement, by amending the Sex Discrimination Act 1975. The definition at the time, found in the amended section 82, was:
"gender reassignment" means a process which is undertaken under medical supervision for the purpose of reassigning a person’s sex by changing physiological or other characteristics of sex, and includes any part of such a process
So the idea of "other characteristics" or "other attributes" comes from the 1999 version of the law. I expect that the change to "attributes" is simply because the Equality Act 2010 uses the term "characteristic" to mean something else, as it consolidates many different regimes about discrimination on the basis of sex, race, disability, etc., all of which are called "characteristics".
The more recent text removes the part about "medical supervision", and explicitly includes people at any stage of transition. Explanatory Notes to the 2010 Act give the example:
A person who was born physically female decides to spend the rest of her life as a man. He starts and continues to live as a man. He decides not to seek medical advice as he successfully ‘passes’ as a man without the need for any medical intervention. He would have the protected characteristic of gender reassignment for the purposes of the Act.
(These notes are not binding in law but they do give a good idea of what the drafters meant to say. The concept of 'passes' is also not the most sensitive language to bring in, since trans people do not necessarily want to measure themselves by their ability to pass undetected, and may present differently in different contexts.)
The example shows that someone can have the protected characteristic without having, or contemplating, any change to their body. So from this example, the other attributes of sex would include all those ways in which the man "continues to live as a man". From general knowledge of trans men, that could include using a masculine name and pronouns; wearing traditionally-male clothes; wearing a binder, baggy clothing, padding in the crotch, or other ways to suggest a more masculine than feminine body; having a traditionally-male haircut; using traditionally-masculine body language; pitching the voice lower; and so on. The situation for trans women would be parallel. These things are all attributes of sex, in that our society abounds in stereotypes of what is a "manly handshake", or a "girly drink", or otherwise associated with a particular sex. They are to do with behaviour more than the body.