KLOS
I am not a pilot. I am a lawyer. Let me try to straighten out a few points:
1. As stated above, the situation you describe has little practical application. If an F/O takes command because he believes the commander is incapacitated, he will certainly have to justify his actions to his employer., but that's where it ends. There will be no prosecution for a criminal offence / involvement of the police / Crown Prosecution Service. Why? Because there is no criminal offence of taking command in case of incapacitation!
2. the defence of necessity that you mention is a criminal law defence. It applies only to certain crimes (e.g. murder). It's not relevant here because of point 1 above.
3. the 'public interest' defence you raise is also irrelevant. That's for public/civil law matters (e.g. newspapers printing stories about celebrities). Again, I doubt the airline will sue the F/O for taking charge, given that this is exactly what he is supposed to do in that situation!
The hypothetical situation you describe is covered by an airline's operating manuals and SOPs. That's the end of it.