Obviously the 50mm is not sufficient to fit a curtain batten. On the face of it I expect it is a mistake. The lintels having been installed one or two courses of bricks too low.
I would expect the solution offered by the housebuilder and warranty provider would be to suggest you fit a ceiling mounted curtain pole!
I have had a quick scan of the NHBC standards 2018 online but cannot find any reference to minimum space from top of window to ceiling. I am sure this used to be 225mm from memory, but like a lot of standards I expect it has been deleted!
What was the situation is the show house and other homes on the development of your house type?
If you can take a photo and prove your home is different they should put this right but how is another matter, fit a smaller window?
I expect the only solution would be to pay you compensation.