Cabin Crew are trained by the airline after successful interview. The training course covers safety, first aid, and cabin service plus a few other bits and pieces.
Safety and first aid are tested and past marks have to be high ie 90+%pass mark.
Training courses last between 4-6 weeks.
There are NVQ qualifactions run by a few colleges (believe Reigate do it)but basically this is a life skills course ie how to get on in the world.
If the crew member fails the tests they usual fail the course and the trainee crew member and airline say Good Bye.