Ours is a very similar size, managed to squeeze a bath where your new shower is and toilet and sink are on wall opposite door which swings towards bath. Only overly tight point is slightly over sized towel radiator that risks burning your knee when taking a dump.

A UFH installation would also help, balls to rad. You can also get a toilet with a basin on top of the cistern lots here https://www.victorianplumbing.co.uk/toilets/cloakroom-toilet.

I would run shower tray along the top wall, with shower at left hand end, then you could have a wider shower, toilet on left wall and basin on right wall. use electric ufh pad under small floor area under tiles, then its easy to replace.

I’d also be perfectly happy embedding the pipes in concrete, hence no issues with running the toilet waste from the door side of the room.
If its deep enough, do 100-150mm stone (40mm to dust) compact it.
set out toilet soil pipe with correct fall.
then 100-150mm concrete. Soil pipe may not be completly embedded in concrete, no issue.
possibly set out shower and basin wastes 100mm insulation
15mm ufh pipes
60-80mm screed on and around ufh pipes
10mm tile adhesive
18-22mm tiles
=400-520mm depth below finished floor level required.

ps I’m not a builder, I’m an accountant who became a vp in a services company who manage and maintain lots of properties among st other things and have overseen lots of kitchen and bathroom restorations.

Wet room?

wet room doesn’t necesarily gain you any space. but could work. the question for me would still eb stopping water getting everywhere. i don’t like curtains.

in your new drawing, could you turn the toilet through 90deg so it backs up to right hand wall, and would you gain a little slace that way for eg the radiator?

From reading the above and the evolution of ideas, it’s a small space so why fill it with bulky items…

Wet ufh
Tiled wet room floor and walls, massive rain shower style shower head
Heated towel rail, hand basin in the back rh corner
Completely open space, no division or curtain
French colonial squat and drop pan, doubling as shower outflow, back left. Easy connection to existing soil pipe.

No personal experience of these other than one I saw in Japan-

https://goo.gl/images/9gkutQ

Saves a bit of space.

Depending upon how wide your door is I think you’ll struggle to fit a long shower area in, you could look at a narrow short bath but I’d think about putting your towel rail on the back wall next to the door. There are compact wc’s and also small basins – look at the Barbican style which is very neat and doesn’t project out too far.
If you tile then consider using an epoxy grout – we have an anthracite grout with swimming pool mosaics which looks great (but cost quite a lot). We also have lime waxed T&G dropped ceiling with a shadow gap.

(The screen contains the water spray and stops the towels and toilet getting wet)