The general rule says 'a' should be used if the 'h' makes a sound, e.g., "a horse" vs "an hourglass", or (somewhat debated) the first syllable is not stressed, e.g., "an hotel".
But I'm not sure how these apply to acronyms - especially for something like HSBC, since people can pronounce the standalone letter 'H' as either 'Eydj' or 'Heydj'
An Eydj-Ess-Bee-Cee branch
A Heydj-Ess-Bee-Cee branch
?