It’s an old terraced house built back in the day when building codes did not require homes to be self-supporting with a gap between them. So the houses actually lean on each other. Whenever a house is removed in my area, huge steel spreaders are installed horizontally to keep the adjacent houses upright.

One idea I have is to just drill a 6mm or 8mm hole and just have a long home-built shaft reach the valve from the bedroom to the shower. the pitfall is that alignment must be spot on precise.