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If (a very big if, coming from a paper) there was zero alcohol in the blood- why did he resign? Would you, if you knew you were completely innocent? I wouldn't. I'd use BALPA and fight.
The captain might have gained something if he was offered his pension and a notice payment, but the FO would have gained little, considering what he has given up.
For both of them, there are various remedies open to them if they are indeed innocent.
If they knew their blood alcohol was zero, and the Norwegians agree, BA have no basis for asking for their resignations and that could easily be reversed- either as part of the internal disciplinary procedure, or later in an Industrial Tribunal. I rather doubt that BA would try to get rid of them if they were not satisfied that the allegations were substantially true.
As I said before, the fact they resigned so quickly tells its own story.