Not unusual to fail the lot, or pass the lot. Look at a standard distribution bell-curve and you'll see why. I think my board had a slightly less than 50% pass rate.
The RN operate a pool system for those who pass the AIB, and you can stay in the pool for twelve months. Each BRNC intake is drawn from those in the top of each pool by specialisation, so you can be in the pool but not go further. To re-join it you have to re-do AIB.
It's certainly cheaper to have an excess in the pool than it is to have spare places at BRNC, particularly as the number get smaller, as the fixed costs of running BRNC haven't shrunk with the reduced intakes.