What would my Higher National Diploma be considered in the United States?
From the website:


  BTEC Higher Nationals are internationally-recognised Higher Education qualifications at level 4 and 5 that are equivalent to the first and second years of a university degree, providing progression to both university and employment.


This means it will probably be considered as something like an associates degree. Some 4-year US universities might give you credit towards undergraduate classes. I doubt any well-respected university would give you credits towards a graduate degree. It is probably insufficient to get admitted directly to a graduate program.