First of all name the housebuilder!
I expect the drawings used were for the terraced versions where these windows would indeed be impossible.
But you have not been given what you were told you are buying and this is not right.
It is perfectly possible to add the missing windows, so the "nothing can be done" comment is factually incorrect!
All that is required would be for scaffold to be erected, and the openings cut with lintels and windows fitted and decoration reinstated. It would be a lot of work for the housebuilder at this stage and cost quite a bit.
Please consider that people would be less likely to buy a home with no bathroom window!
You will also have problems with condensation, possibly mould, even with an extractor fan.
The "error" also demonstrates a lack of attention to detail, something you should also be concerned about. Please get your new home professionally snagged and inspected before you legally complete.
I recommend Tim Fee 07768 900200 from Snagging Inspections.