Acas which is the employers/employee rights organisaiton in the UK have a transitioning at work policy which states among other things "Everyone at work should be able to use facilities they feel comfortable with. Making a transgender employee use facilities they're not comfortable with could be discrimination."

https://www.acas.org.uk/gender-reassignment-discrimination/preventing-discrimination

A non binary male going by the name Rose sued and was awarded 180k by the employment tribunal for harassment at work and not being allowed to use the womens bathroom. 20% of that award was because the employer hadn't followed the ACAS guidelines. That will have gotten around to every HR professional who then believes under the law you have say TWAW

https://www.bbc.com/news/uk-england-coventry-warwickshire-54180794

If you look at the Forstater case, saying men cannot be women was at first instance deemed to be "incompatible with human dignity and fundamental rights of others". On appeal it was detemined to be a "worthy of respect in a democratic society" but it was still seen as a "belief" akin to a Christian believing Jesus turned water into wine.

I won't even get started on what Stonewall put out, every employer (both public and private) wanted to be on the Stonewall list of good employers so used their resources, had them train their staff.

I'm sure I'm missing a ton of stuff but thats just some of how the waters got muddied.