A failure of IT systems shouldn't stop the plane flying... The gate staff should just let anyone onboard with a legit ticket or a sufficient story about buying one. Sure, some people might get a free ride, or a few legit ticket holders can't get on because the plane is full, but overall, most passengers end up happy. Simple things like "the pilot can't log in to the HR system so he gets paid for the flight" should be…
In yesterday's world, yeah. Today, when you have to take off your shoes to get onto a plane in the US, I don't see that happening.
If BA put aircraft in the air without any idea who was on them, they’d be rapidly forced to land again with the threat of being shot down.