I'm actually not sure why an employers shouldn't be able to decide he doesn't want men wearing dresses made for women, or presenting as women.
They would probably fall foul of the EA2010 if tested in court.
Office Dress Code UK: 2026 Rules, Examples & Employer Guide
5. Sex, gender and office dress codes
Some dress code policies set different appearance standards for men and women. Historically, this has been permitted where the overall standard imposed on each group is broadly equivalent. Case law, including Smith v Safeway, is commonly referenced in relation to the principle that employers may apply different requirements provided the overall burden is comparable and not more onerous for one sex.
However, employers should take care that any differences do not place a greater burden on one gender. For example, requiring one gender to wear specific items of clothing that are uncomfortable, impractical or more expensive may create a risk of discrimination claims. Employers should also take care to ensure dress code standards do not adversely impact employees undergoing gender transition or those who do not identify with gendered clothing norms. Employers should be mindful of potential risks relating to gender reassignment discrimination where policies are enforced inflexibly or in a way that targets an employee’s gender identity.
Many organisations now adopt gender-neutral dress code policies, focusing on the standard of dress rather than prescribing specific clothing based on gender. This approach can help reduce legal risks while giving employees greater flexibility in how they meet workplace expectations.
https://www.davidsonmorris.com/office-dress-code/
"individuals who cross-dress without intending to live permanently in the gender opposite to their birth sex are generally not covered under this protected characteristic." (Gender Reassignment)
https://www.davidsonmorris.com/gender-reassignment-discrimination/
Even if there was no PC of GR it could be the same outcome under the PC of Disability and/or Belief.
Equality Act 2010: Draft Code of Practice for services, public functions and associations, 2026
Published 21 May 2026
Gender reassignment
2.46 A person who dresses and presents in ways which are commonly associated with the opposite sex will be protected where they do so as part of a process of reassigning their sex. The Act does not define what is meant by reassigning sex. It does not have to be a medical process – though some people will take medical or surgical steps – but it is likely to at least involve more permanent changes, such as changing one’s pronouns and consistently dressing and presenting as the opposite sex. A person presenting in a gender non-conforming way for some other reason does not have the protected characteristic of gender reassignment. However, they may be protected against harassment and discrimination by perception (read paragraph 4.51 to paragraph 4.53).
Example
2.47 A group of men who have dressed in women’s clothing for a stag party are refused entry to a nightclub that has a dress code. This is unlikely to be discrimination based on the protected characteristic of gender reassignment.
2.48 Where an individual has gender dysphoria and the condition has a substantial and long-term adverse effect on their ability to carry out normal day-to-day activities, they will also be protected under the disability discrimination provisions of the Act (read Chapter 6 and Chapter 7).
Manifestation of religion or beliefs
2.82 The protected characteristic of religion or belief includes protection for the manifestation of a religion or belief. This protection is consistent with Article 9 of the European Convention on Human Rights, which guarantees freedom of thought, conscience and religion.
2.83 Manifestations of a religion or belief could include:
- treating certain days as days for worship or rest
- following a certain dress code
- following a particular diet
- expressing religious or philosophical views online
- carrying out or avoiding certain practices
2.84 A person does not have to prove that the manifestation of their religion or belief is a core component of the religion or philosophical belief they follow. The manifestation may be a means by which they choose to express their adherence to their religious belief. However, there must be a sufficiently close and direct connection between the manifestation and the underlying belief [footnote 11].
2.85 Placing limitations on a person’s right to manifest their religion or belief may amount to unlawful discrimination. Read paragraph 4.91 for more detail.
https://www.gov.uk/government/publications/equality-act-2010-draft-code-of-practice-for-services-public-functions-and-associations-2026/equality-act-2010-draft-code-of-practice-for-services-public-functions-and-associations-2026
The Code of Practice for Employers is going to be much the same when it is issued.