Not unthinkable at all. There is a splash back product called metaline by laminex that is a splash back surface, with benefits that glass doesn't have. Firstly, the cost factor is reduced as it can be installed as per simple instruction with adhesive and silicon sealant and can be post cut meaning if you later decide you want to install power points or cut outs for taps, lights etc, you can cut through it on site after it's installed, with rouge Ed glass for example that cannot happen. Secondly, it is a large scale product without pattern, so joins are minimized although some metallic colours are in the palette so they can be reflective and moody if that's what your interested in. Metallics are right on trend, so one of their neutral tones would line the entire bath feature wall and look sophisticated and not fussy at all. It could also provide the colour inspiration for the room, if everything else is kept neutral. Go online for it's use and samples. Good luck. ML Design

This with the extra width you suggested the built in bath with a large shower space next to, the shower head coming off the opposite wall to help avoid water to window, the window would be best tiled, the wall with the shower door into "The wet room could be all glass up to the vanity. There are several suitable places for towel hanging and an extra storage cabinet could be above the toilet, I forgot to add this

my first suggestion has minimal plumbing changes and no wall changes although i would reverse hinges on the entry door, space for a 90 x 90 shower and rotating the toilet should be quite straightforward for a 730 wide toilet space ( a little tight but adequate for most people especially if you're ok with a 620 door) a standard 1500 x 450-500 vanity will fit under the window and the gap at one end is ideal for hanging a hairdryer with a handtowel rail higher up the wall, there's wall space next to the shower for a bath towel rail and a mirror doored wall cabinet would be ideal...and, for a little more expense another option with a double shower.. a relative term with bathrooms that are always more expensive than expected but well worth obtaining quotes to find out costs for both options for comparison..ie do you want a generous shower or vanity? btw i think moving walls in a solid brick house has potential unwelcome surprises and doesn't improve options and installing different fixtures will need new flooring and waterproofing anyway so no better time to do a full upgrade and changes to the floor is the best way to achieve a walk in shower but you need to discuss with a plumber before making any decisions

@Kate We were thinking that having the shower rose on wall B might help reduce the splash out. @bigreader The opening is 400mm, but we were thinking 500-600mm would be a better option. A shorter panel might increase splash zone? @dreamer We were aiming for a 600mm entrance. Extending the vanity across could be an option (the current layout is showing a 600mm vanity) - this might give us more space for the toilet (we are not bigger than average) or a larger vanity. @K s Could certainly be an option - we just thought with this, it would save the first thing you saw when opening the door from being the toilet. We were thinking to attach toilet roll to wall behind toilet or on the vanity itself.