BA don't fail applicants to balance the numbers, so there isn't a failure rate as such. If the 30,000 that applied were *all* suitable then they might all get a job, however BA has a need for only x number of applicants, so it will offer positions to the first x hundred that make the grade. It is not a lottery as such where you are a statistic, if you're up to BA's spec, you'll get in, if you're not, you won't. Simple.
When I had my DEP interview, BA told us all at the interview that if all 30 odd of us were up to it, we'd all get offers. There is no competition between applicants just a limited number of places each year.
Good luck!
Rod