Do British English-speakers call American Lieutenants 'lef-tenants' or 'loo-tenants'?

In British English, the military (and police) rank of ‘lieutenant’ is pronounced ‘lef-tenant’. In American English, it is ‘loo-tenant’ (approximately).

Are these treated as two distinct pronunciations of the same word, or as subtly different words? E.g., if a Brit is addressing a US Army lieutenant, should they say ‘lef-tenant’ or ‘loo-tenant’ (and vice versa)?