Should we say “an NHS doctor” or “a NHS doctor”? There are two schools of thought on this subject: “vocalisation of the abbreviation” and “vocalisation of the first word”.
By this rule, if the abbreviation starts with a letter that starts with a vowel when vocalised, the article used is “an”; if it starts with a consonant-sounding letter, it takes an “a”. This would give “an NHS hospital” and “a BBC documentary”.
Vowel-sounding letters (take “an”): A E F H I L M N O R S X
consonant-sounding letters (take “a”): B C D G J K P Q T U V W Y Z
(Note: H is pronounced “aitch”, not “haitch”, although this rule might be becoming more fluid.)
By this method, the first letter of the first word as it is read out determines whether “a” or “an” is used. Here we would have “a NHS hospital” (“a National Health Service hospital”), or “an UV lamp” (“an ultraviolet lamp”).
The second method can look and read quite awkwardly but is just about acceptable as long as the style is used consistently, or where abbreviations are commonly fleshed out in the head and in speech. (For the record, I don’t think either of the examples here qualify.) Since the first method reads more naturally in the head, it’s easy to slip into it when the second style is supposedly being observed. The first method is by far the more popular.
An acronym is an abbreviation that is read out like a word, such as NASA, OPEC, NATO, etc. So here we would always have “a NASA spacecraft”, etc. These will always be treated as words, not abbreviations, so will always take the appropriate indefinite article.