Your question doesn’t make sense in terms of UK toilet regulations and legislation. If the toilet is mixed sex design, it has to be self contained and completely private. It’s supposed to be single use, separate from everyone at point of use.
In terms of safety, this is not the safest design. At a club a woman had choked on her own vomit and was on the floor on her cubicle. My friends and I saw her immediately when we entered the ladies as her blue hand was visible from the floor to door gap. We got over the door, opened it from the inside (made more tricky as her body was in the way) pulled her out, and got the vomit out of her mouth and whacked her on the back til she started breathing again. In a mixed sex design I really don’t think she would have survived. She wasn’t fully conscious when the paramedics took her away but she was choking up sick on her own and it was obvious from the smell she’d had too much too drink.
The above situation could be drugs, a mental health crisis, a heart attack, stroke, hypo, seizure etc etc.
Its the fact that you, as an individual, are visually and audibly separate from everyone else who could help you that makes it so unsafe in a mixed sex toilet.
Also in terms of safety comes health. You can catch viruses and bacteria from dirty surfaces and air. Mixed sex toilets have the greatest number of pathogens.
I haven’t even mentioned assaults (as other posters are covering this) but mixed sex cubicles are less safe too due their private design. Women scan the environment more than men. It is useful to have an idea of who will be outside the cubicle too which can’t happen in mixed sex toilets.
The safest toilets are those that are single sex with a single sex area in front so the cubicles can have door gaps for health and safety.
If you come back OP, I would be interested to know your thoughts on toilet design regarding safety and separation.