For the purposes of the EQUALITY ACT which is written to include ALL people in society to live as freely as possible, all men and transwomen, regardless of their GRC status remain male.
If there is a legitimate aim, males can be excluded from certain roles and situations to ensure that females feel safe, comfortable and retain their dignity.
A job working in a women's lingerie department potentially can use THE LEGAL EXEMPTIONS of the act for the legitimate aim of ensuring their customers feel safe, comfortable and retain their dignity rather than self exclude from the necessary task of shopping for a bra. The transwoman does NOT have a necessary need to work in that department and can be tasked elsewhere.
The key point is which group has a NEED here.
Living within a society DOES NOT mean we all have access to everything. It means we are free to live within certain limitations and this applies to us all. We can not do certain things just because we want to because they are antisocial and have a negative impact on others. For example someone might want to play loud music at 4am but they are not permitted to do so, because it's not very fair on others. They can usually crack on playing the music in the middle of the day as long as it's not persistent and regular.
The equality act takes this principle to the next level in terms of our personal characteristics and what is fair. What is fair does not mean you get to do everything. There is a difference.