I once tried to check in at EDI 3.5 hours before the flight and was turned away for being too early.
I questioned the logic of me being turned away if I could just PYOBP and just wander straight in. The queue dragon then suggested that I went to an internet cafe to get a BP at my expense.
In the end I just went to customer services and told them my printout didn't work and they just gave me a BP on the spot.