I am not posting this because I think the advice in the article is 100% correct, but more to show that recent media coverage and legal actions, has meant that mainstream employers are having to (re) consider their toilet policies!
Providing single-sex facilities is an aspect of work where de facto discrimination is potentially permitted under the Equality Act 2010. Preventing an employee from entering facilities allocated to the opposite sex is unlikely to amount to sex discrimination under the Act in most cases.
Indeed, the Workplace (Health, Safety and Welfare) Regulations 1992 set out statutory obligations to provide separate male and female toilet facilities for employees. The regulations do allow for facilities not to be provided for different sexes, but only where they are in separate rooms that are lockable from the inside. In Miller, the claimant successfully argued that the facilities she was provided with were inadequate, in part because they exposed her to the potential discomfort of sharing that facility with a male colleague using the urinal.
That case involved a cisgender female employee who, by virtue of her sex, held a protected characteristic under the Equality Act 2010. However, the outcome of the case may impact employers faced with the already difficult issue of providing appropriate facilities for transgender employees.
https://www.peoplemanagement.co.uk/article/1822248/considering-legalities-single-sex-toilets
Although the final paragraph certainly seems to imply the writer of the article thinks the impact on trans people is what should be upper most in employer's minds.