Sorry for brief reply previously, was not able to spend more time.
The room has always, since it was built in 1850, been a "toilet". The outside drainage pipes look original and I suspect the connection to the downpipe is lead. It is in a wedge-shaped room at the end of the hall on the gable end. I've no idea how it was originally fitted out but I believe the standard flushing toilet was in the same position it is today, facing the door, looking down the hall. The triangular floor area to the right is moderate sized and at some time has had a cast iron roll top bath. I suspect this was added after the original construction as it only just fits. The wall at the narrow end has had its plaster removed to allow it to fit.
The taps are on the wall over the bath. Also, the bath drain has been added later. The current pipe is plastic but this could be a replacement.
The shower head looks like it always fitted in the lath and plaster ceiling and I can't see any sign that it was ever different. I wonder if there used to be something else below to catch the water. I don't know how it would have been drained. The paint is very multilayered so I suspect the shower hasn't been used for many decades. The room has a wooden floor and I see no evidence of any tiling or "wet room" type approach. Puzzling. It's possible something was removed when the bath went in.
I think the general room space has been always thus. I also know that it was used in the last decade.
There is an ancient water tank above, which looks pretty ropey. Goodness knows how I will get it out to replace.
A bit of googling led me to this image (below) of a shower head described as "1850's" so it seems plausible that this could be an original 1850's shower head.
Our plan is to convert a small adjacent room to the new bathroom and keep the small toilet. I plan to use this space where the bath is for the new water tank, boiler and shower pump etc.