I think it has one of those door doors just put of shot. When house buying I.saw a few that had converted the lean-to into a toilet and the upstairs toilet into another room. Some had this configuration with a door to it from the kitchen. One had you going outside to use it and, considering it had the only bath and shower in the house, that'd have made for an invigorating stroll during Winter.
It wouldn't slide to the left (in the picture) because there are electrical outlets and lightswitcbes in that wall. It might slide to the right, or there might well be a regular door just out of sight on the right.
This is common in old houses. I don't know if that's the case here. Houses built before indoor plumbing would be upgraded, the kitchen would get water, and a new bathroom would be built next to the kitchen where the new plumbing was. There would be no water or other bathroom in the entire house.
My grandparent's house was built like this, they still had the old water hand pump in the front yard, and they had an outhouse until the plumbing was upgraded.
Berlin in the 80s had shower cabinets in the kitchen and not walled off. Because the old apartment buildings didn't have any bathrooms, toilets were shared between all apartments of a single floor. So when it these shower cabinets became available people put them into their kitchen.
My experience with toilets in the UK is basically crap, crappier, crappiest. Either dirty, malfunctioning, in a very odd location (once stayed in a place where you couldn't close the door if you sat normally on the seat, so you had to shit sideways), or any combination thereof.
A lot of houses in the UK are old enough to have had an outhouse, and bringing the toilet back into the house means it's in an odd place, or the bathroom ends up more cramped than is ideal.
In houses I've been in before with similarly places bathrooms, this halfbath is generally for guests, peeing, and basic washing of hands etc. If you're going to blow it up, that's for other places.
You want everyone washing their dirty hands in the kitchen sink?
Your aren't, it's all good. I'm just wondering what problems someone could have with this setup. Nothing comes to mind other than not liking small bathrooms.