What impact would a Brexit have on employment of non-UK residents?
Non-UK residents who are UK nationals: I can't see there being any reason for any change...fortunately most countries allow their own nationals to work within their own borders.
Non-UK nationals, regardless of their country of residency: Nobody knows.
IMVHO and as far as BA is concerned my
guess is that if it's "leave", eventually and after negotiations there will be "grandfather rights" for those non-UK nationals already in BA to allow them to remain employed. For new recruits I'd hazard another a guess: based on the demand for employees and the numbers of suitable applicants within the UK and elsewhere BA will try very hard to ensure the UK government issues work permits/visas for those from the EU (and perhaps elsewhere) who want to join BA.
Knowing the way BA work I certainly cannot see them meekly accepting they can only recruit from the UK labour pool..
https://en.wikipedia.org/wiki/Grandfather_clause