From their style guide:

"Where you would normally pronounce the abbreviation as a string of letters - an initialism - use all capitals with no full stops or spaces (eg FA, UNHCR). However, our style is to use lower case with an initial cap for acronyms where you would normally pronounce the set of letters as a word (eg Maga, Sars, Aids, Nasa, Opec, Apec)."

(Under "acronym": https://www.bbc.co.uk/newsstyleguide/a )