The Queen’s English, since “American English” is technically older and more proper(if not as nice), spelling notwithstanding. Also, “an HD” manages to be both phonetically cleaner and faster, reducing the time needed to communicate. It wins out on that alone. :D

Wrong, the word doesn’t have to start with a vowel, the first syllable has to be pronounced as a vowel. HD starts with a vowel syllable, therefore ‘an’.