The current norm, which GC people want to change, is that transgender people use the toilet which matches their own gender.
I would suggest it is possibly the norm in businesses which have paid to be part of the Stonewall Employer Diversity Scheme.
The fact that it is the norm amongst such employers does not mean it is the norm elsewhere. As pp says most of the employers I've worked with are not part of such schemes and have either single sex loos or for very small employers mixed sex loos.
The fact that it is the 'norm' for Stonewall scheme employers doesn't mean it would withstand scrutiny should a female employee make a request to an employer for single sex provisions, particularly if that female employee had specific reasons for not being able to be in mixed sex provisions such as because of religious belief or being a sexual abuse survivor. The larger the employer the more likely that they would discriminate against the female employee for not making single sex provisions, especially given that they are specifically provided for under the EquA.
So please do not assume that because Dadjoke says it's the norm you have to put up with it if your employer fails to provide single sex spaces, or if you are in HR don't assume you have to make all single sex spaces mixed sex. This is completely and utterly untrue.
The LAW as it currently stands allows for single sex provisions if it is legitimate and proportionate, it is hard to see how single sex toilets in the workplace fail to meet this. Obviously if you have a TW employee you should be sensitive to their needs and it would be appropriate to offer an alternative to the male facilities where possible.