Can a man claim he has the right to be in the women's toilet or changing room?

Customers aren't liable but a shop may be if failure to provide a single sex space means women are less able to access their service.

No, customers are not liable under the Equality Act, or responsible for implementing a provider's policies.

I suppose the customer could be asked to leave, on the grounds that they have breached the shop's policy, in the same way that I could be asked to leave a shop or cafe for all sorts of legal but disruptive behaviour. The way I see the Equality Act operating here is that, if the man said "You've discriminated against me based on my sex, because you wouldn't have thrown a woman out of the ladies' loos", the provider can say "We haven't discriminated unlawfully, because we are making use of the single-sex exception here, in a way which is reasonable and proportionate".

However, if it is a changing room where women are undressed?

I doubt it (unless civil trespass laws) but if they refuse to leave when requested they could be. Especially if they become belligerent. A bit like I wouldn't be breaking the law if I decided to wonder through to the loading bay of the supermarket.

IADNAL. But since the provider of services/facilities can legally designate their service/facility as single-sex, provided that doing so is "a proportionate means of achieving a legitimate aim", isn't anyone ignoring that provider's decision breaching the law, specifically the Equality Act 2010?

Quite, they should be labelled mixed sex

I think the onus should be on the business to label their toilets and changing rooms accurately.

This. I don't understand how claiming something is single sex when they aren't making any reasonable attempts to make it single sex isn't against trades discription act.

I don't know whether this is aimed at me. As a small-l liberal, I think I agree with you.

It's a bit like trespass: it's not illegal to enter someone else's property, it is illegal to cause harm or the fear of harm to the property or to anyone in the property.

Toilets are single-sex in order to prevent harm, or the fear of harm, to women.

IDK if you are misunderstanding my posts, or we are just coming at this from different angles. I don't think it is or should be illegal to walk into or use the wrong toilet.

Men can legally be asked to leave/be excluded from toilets/changing rooms and the police called if they refuse to go (in Scotland too) not by individual service users but by service providers - local authority leisure centre owners, gym owners, etc ('owners' being represented by their staff or volunteers). If they don't then individuals may be able to make a claim for sex discrimination against that service provider as single sex facilities are provided to enable women to access facilities.

This may not be helpful but I understand the situation in general is that you may do anything not specifically illegal (although efforts might be made subsequently to make your actions illegal if necessary). The way you are framing the question applies to criminal behaviour, though. The way I see it, lawful/unlawful discrimination is a civil law matter. The default position is that we all have the right to be everywhere in public spaces. If a woman enters a space which has been designated male SS by the person/authority entitled to do so, she isn't committing a crime but she isn't entitled to be there because of the designation of the space as male SS by the authority - who are permitted by law to curtail her right to go anywhere. If the space was not designated male SS, it would not be lawful to exclude her. But the authority wouldn't be committing a crime by excluding her although she might be able to sue them, depending on the specific circumstances.
The right to go anywhere is the default for everybody but that right can legally be restricted for reasons and in ways set out in the Equality Act. So I would still say that no-one has the right to be in areas which have been set aside for the opposite sex by the person/body authorised to do so. If the authority gives permission or hasn't designated an area SS, then the default position applies.
Again, IANAL!

Sex segregated or single sex spaces should mean no adults of the opposite sex allowed entry. It should be considered an unwritten rule even there is an actual law or not.

Same discussion in 2018
www.mumsnet.com/talk/womens_rights/3140457-Is-it-legal-for-a-man-to-use-the-womens-toilet-in-England-Scotland

And interestingly a technical review following a call for evidence which I wasn't aware of:

www.gov.uk/government/consultations/toilet-provision-for-men-and-women-call-for-evidence/toilet-provision-for-men-and-women-call-for-evidence

Not any more... women means feelz to most organisations now

I meant to add: 1) Single-sex services/facilities are vastly preferable and should be the default; 2) Mixed-sex ones should only be offered in circumstances where provision of single-sex services/facilities is untenable due to genuine constraints (e.g., where a tiny café only has space for a single lockable toilet, etc.; 3) When it comes to serious matters, such as DV refuges, rape counselling, prisons etc., provision No. 2 above should be entirely disregarded and the financial/space constraints fully mitigated so that single-sex provision is possible.

The sign on our loos just has someone in a skirt. There are no men in the ladies loos where I work but there are a couple of mildly militant, blue haired women who are currently using the mens loos. Maintenance have blocked off the urinals. There is a decent amount of ffs and eyerolling from the blokes. Been Stonewalled so nothing is said overtly but still made clear.

This! As to the OP's question, no a man can't claim the right. In the absence of any other evidence, such as clear signage on toilet doors that they are mixed sex / gender neutral, men should use the facilities appropriate to their sex. If he is unhappy, at present he could check with the provider of the services and facilities, since they can legally insist on single-sex provision but regrettably do not always do so. It is sad that the EHRC guidance on this issue is not more robust, but hopefully we are s-l-o-w-l-y getting there.

I don't agree that the question has no meaning, or that it's entirely down to social convention.

Social conventions inform the way that the law is interpreted. For example, I have the legal right to walk down the pavement. If, however, I pace up and down the section of pavement outside your house all day, this might be used as evidence of stalking, harassment or similar. This is probably a rubbish example, but you get my drift.

I think it's a very good question, seeking to analyse the logic of the jumbled legal landscape where sex and gender are dealt with.

I agree nobody ever thought a law explicitly forbidding men's access to women's loos would be necessary, and that such a law (now) would be a blunt instrument ready for self-identified lawyers, lobbyists and litigation enthusiasts to beat us over the head with.

So I think you would have to make a case to the police that the person involved is 'acting in a manner likely to cause fear, distress, or alarm' and hope that the police would view it as a public order offence.

Well, I think in a sane world, it would be universally accepted that a man in the ladies' is doing exactly that, just by virtue of being there. Unless, obviously there were extenuating circumstances. The intended purpose of a ladies' loo is to provide toilet facilities for ladies.

Sadly, we do not live in a sane world.

If the sign says 'women' its reasonable to expect a single sex service.
If its not single sex they need to change the sign.

I've thought about what would happen in the event you wanted a male ejected from a women's loo. I'm in Scotland, so I only really think in terms of Scots law.

If the staff of the facility were unwilling to intervene and the male is adamant on his right to use it, presumably you would call the police. As there is no law that prohibits people from using any bathroom they see fit, it is just social convention and people's willingness to follow this that means males typically use their own loo, women theirs'. So I think you would have to make a case to the police that the person involved is 'acting in a manner likely to cause fear, distress, or alarm' and hope that the police would view it as a public order offence.

However, the pendant in me thinks that provided the person in question was doing nothing more than using the loo for it's intended purpose, I don't see how the police could in any way view that as 'acting in a manner likely to cause fear, distress, or alarm'. I mean, they are using the facility for it's intended purpose. Going to the toilet is not a public order offence.

In short, I think a man absolutely can use a ladies loo, and there isn't a damned thing that can stop him provided he is genuinely using it for it's intended purpose.

I've used disabled loos in the past despite not having any obvious outward disability. I know it's not strictly the same thing, but if someone else challenged me on my use of a disabled loo, I would not, as a first recourse, immediately reveal my hidden disability to a total random stranger. I really do not need to 'prove' my entitlement to use the facility. If they believe I am not entitled, then they can call the police and make their argument as to why. Aside from the fact I am indeed perfectly entitled to use a disabled facility, I'd love to know what sort of argument they'd make to say that I'm not. After all, what can you possibly be doing wrong by using a toilet for it's intended purpose?