The Equality Act says:
"This paragraph replicates the effect of exceptions for occupational requirements in current discrimination legislation, and creates new exceptions in relation to disability and to replace the existing exceptions for occupational qualifications in relation to sex, gender reassignment, colour and nationality. It differs from the existing exceptions for occupational requirements in that it makes clear that the requirement must pursue a legitimate aim and that the burden of showing that the exception applies rests on those seeking to rely on it."
"Examples
The need for authenticity or realism might require someone of a particular race, sex or age for acting roles (for example, a black man to play the part of Othello) or modelling jobs.
Considerations of privacy or decency might require a public changing room or lavatory attendant to be of the same sex as those using the facilities.
An organisation for deaf people might legitimately employ a deaf person who uses British Sign Language to work as a counsellor to other deaf people whose first or preferred language is BSL.
Unemployed Muslim women might not take advantage of the services of an outreach worker to help them find employment if they were provided by a man.
A counsellor working with victims of rape might have to be a woman and not a transsexual person, even if she has a Gender Recognition Certificate, in order to avoid causing them further distress."
These examples appear to relate both to sex and gender reassignment, i.e. it is clear that you can exclude a transwoman from a rape counselling job, but it is not clear that you can exclude them from a women's toilet attendant job (the example appears to refer to sex rather than gender reassignment).
These are just examples and the Equality Act doesn't really say any particular job is reserved, it's all subject to reasonableness.