It is required for flight time limitations on certain trips. Not that often for cabin crew though who have less restrictive hours limitations.
In the case you outline, in all airlines -
a) flight continues if the captain thinks it's safe, prudent, and legal
most airlines
b) that's it
BA
b) Crew call BASSA on landing
c) BASSA complain to BA
d) BA call Captain for an explanation
e) Cabin crew get hosed down with cash