You really need to name the housebuilder.
Yes the NHBC have the
Consistent approach to finishes which is the housebuilders 'get out of jail free' card. So much for the NHBC's clam of "raising standards" and "protecting homeowners"
If a floor is not flat as opposed to not perfectly 100% level, it is wrong.
The housebuilder should be doing it again, not trying everything possible to wriggle out of their bad workmanship, possibly with the aid of the NHBC.
If any of the NHBC "guideline" tolerances are not met, then the floor should be done again.
Get the NHBC to inspect it anyway.