I agree with you both, which is where the weakness in the cabin crew argument will be.
All the CAA and BA would have to prove is that the Cabin Crew used their security pass to gain access to a restricted area, and the purpose for doing so was to perform their duties as air crew which they subsequently failed to do. All very easy to prove. BA can then contest that there was no other reason, other than the strike, for them to fail to perform their duties.
This leaves one of two scenarios. If, as you suggest, the cabin crew passed to the secure area without the knowledge that they were going to be called out on strike, and subsequently were by the union it is a safe assumption, because of the timings involved, that the union knew the staff were airside and yet had no intention of letting the individual fly. Individual would still be liable as they can not put union position ahead of CAA regulations.
Other scenario is that individual passed through the secure area with no intention of undertaking their duties. Individual is liable.
The ramifications could be potentially be exceptional for the individual, as CAA has the authority to remove their security access at any point they are given cause to, so it is not a course of action that should be treated lightly.