Emily, with being a renter, I'd definitely check Pinterest for temporary solutions. I've seen DIYs where you can starch fabric to walls with an iron. Also try temporary wall paper/decals, but I'd test a small area first. Cabinet makers or even yourself would be able to create a flush vanity without attaching it to the wall. Plywood, especially hoop pine, has received a lot of traction. It really gives a clean, organic look that you could have custom cut to fit in the area. You could also use it on the wall – it could be stained, left natural, or painted for a feature. Have a look at EchoPanel. It is a decorative, acoustic pinboard that would add a splash of color/design. If it is above your vanity, you could pin pictures, inspirations, etc.

My first rule for bathrooms is to make sure the toilet is not the first thing you see when you open the door. Move the toilet to where you currently have the shower, backing up against the wall with the door. It's also nice to have the toilet near a window! If you can, convert that door to a cavity sliding door to increase available room in the bathroom itself. A shower along the wall where you currently have the bath would have a number of benefits - it has a window, which is great for drying it out and preventing mould, it is long enough to have a towel rail at the end of it without towels getting wet and you wouldn't need to have a door on the shower (saving space again). I would suggest the shower rose go on the left end of the shower and the opening (in lieu of door) and towel rail at the right end. I would position a wall hung vanity unit on the left wall where you currently have the toilet. Good luck!

Can't see the room layout, but if you keep it, it could be a nook for a sofa or chairs by the fire on a cold night, with a rug in front. Definitely have a mirror on the mantle. Think about walls and rooflines before you do anything. Or if you've got lots of cash to splash, take it out as it may serve no purpose with a heatpump in place. Replace roof tiles, internal brickwork.........you see what I mean, just saying......

It looks wonderful Jo. I love that you've used the same tiles for the feature wall, it looks cohesive and the space appears larger. Any other feature tile would have overwhelmed the small space.