However, I'm still no further forward. Some laws appear to conflict each other, so it would be difficult to complain about this based on that - I could quote the Equality Act 2010 and the HR department could say 'Ah, but...' and how can I respond when it's all so woolly?
Don't let the be-kind-brigade confuse you, LockdownLisa. The law is clear on toilets in the workplace. (Services are a different matter)
The relevant law here is the already mentioned The Workplace (Health, Safety and Welfare) Regulations 1992, paragraph 20 which states:
Sanitary conveniences
20.—(1) Suitable and sufficient sanitary conveniences shall be provided at readily accessible places.
(2) Without prejudice to the generality of paragraph (1), sanitary conveniences shall not be suitable unless—
(a)the rooms containing them are adequately ventilated and lit;
(b)they and the rooms containing them are kept in a clean and orderly condition; and
(c)separate rooms containing conveniences are provided for men and women except where and so far as each convenience is in a separate room the door of which is capable of being secured from inside.
(3) It shall be sufficient compliance with the requirement in paragraph (1) to provide sufficient sanitary conveniences in a workplace which is not a new workplace, a modification, an extension or a conversion and which, immediately before this regulation came into force in respect of it, was subject to the provisions of the Factories Act 1961, if sanitary conveniences are provided in accordance with the provisions of Part II of Schedule 1.
Part II of Schedule 1 explicitly mentions female-only and male-only toilets making it clear that men and women are defined here in line with the usual legal meaning of males and females.
www.legislation.gov.uk/uksi/1992/3004/regulation/20/made
Please note that unless your employer provides separate rooms for each toilet (this must be a room fully enclosed by building walls, not cubicle walls, but sinks may be shared), they are not complying with the law if they allow your male colleague to use the female-only toilets.
The Equalty and Human Rights Commission are the regulator enforcing the Equality Act 2010. Their guidance is statutory (legally binding), and this is what it says on the issue:
17.55
If a worker is undergoing gender reassignment, it is good practice for the employer to consult with them sensitively about their needs in the workplace and whether there are any reasonable and practical steps the employer can take to help the worker as they undergo their gender reassignment process. For further information on gender reassignment, please refer to paragraphs 2.21 to 2.30 and 9.31 to 9.33.
Example:
A worker will soon be undergoing gender reassignment treatment and the employer has accepted that they want to continue working throughout the transition process. To avoid unresolved questions about which toilet facilities the worker should use, their uniform and communications with other members of staff, the employer should arrange to discuss the situation sympathetically with the worker. The discussion could cover setting a date for using different facilities and uniform; the timescale of the treatment; any impact this may have on the worker’s job and adjustments that could be made; and how the worker would like to address the issue of their transition with colleagues.
From Employment Statutory Code of Practice, page 254
www.equalityhumanrights.com/en/advice-and-guidance/equality-act-codes-practice#h3
You'll note that the code does not say that the worker must be allowed to use opposite-sex facilities, but merely "different facilities". This is because an employer must make reasonable adjustments for the worker, which can be a gender-neutral toilet or a disabled toilet (the code specifically refers to such a person also having the protected characteristic of disability if they are medically transitioning). But because The Workplace Regulations apply, they cannot simply insist that this worker should use opposite-sex facilities, because that would break the law.
Now we had a PP earlier quoting from guidance produced by the Government Equalities Office called The recruitment and retention of transgender staff. Guidance for employers. This was published in November 2015. What's important here is that this is not statutory guidance and it cannot supersede the Equality Act 2010, the EHRC Code of Practice or the Workplace Regulations. (The GEO have confirmed elsewhere that where a single-sex provision is a proportionate means to a legitimate aim, it is lawful to exclude people who have the protected characteristic of gender reassignment.)
www.gov.uk/government/uploads/system/uploads/attachment_data/file/484855/The_recruitment_and_retention_of_transgender_staff-_guidance_for_employers.pdf
Now for the Equality Act 2010. This gives you the protected characteristic of sex and contains various provisions intended to protect you from unlawful discrimination in public life. Which includes your employer. The Equality Act 2010 explicitly allows single-sex provisions where a member of one sex may reasonably object to the presence of a member of the opposite sex. It also lists the protected characteristics of disability, religion or belief and race or ethnicity. Which means that some female employees can have additional protected characteristics and an employer must ensure that they do not unlawfully discriminate against all of these protected groups, whether directly or indirectly, and not just cater to your colleague.
So what can you do?
I would follow the advice provided by others here and ask your employer to please signpost you to the female-only toilets that they are obliged to provide for the privacy of female staff. I wouldn't justify it or ask about their policy in regard to staff who identify as trans, because that's not relevant to you and they'd probably just send you waffle anyway. All you want to know in your first step is where the single-sex toilet is for female staff (not just for you, but all female staff).
In your second step, you can list the groups of women who will be excluded from the previously female-only toilet by making it single-sex and argue the law. But for starters I wouldn't go in guns blazing - you know they must provide either female-only multi-entry or unisex single-entry toilets by law, so just send a matter-of-fact enquiry as to where those are. If they say there aren't any, send an email asking them to confirm if you have correctly understood that they no longer provide female-only toilets and that all toilets provided for the use of female staff are instead now mixed-sex (using those words).
And then I would argue the law with an emphasis on women being discriminated against by denying you the female-only toilet you are entitled to. (But only if you feel comfortable doing so. There are other ways to approach this if you are worried you will be penalised for asking the question.)