FRying,
Under UK EMPLOYMENT LEGISLATION, UK pilots CANNOT refuse to operate the French flights! To do so would open themselves up to possible dismissal for breach of their UK contract and if they are a BALPA member could expose the union itself to court action. This is the official line from BALPA.
I'm not sure many of the crews operating ex CDG/ORY today are "happy" to be doing so but they simply have no option.
As I understand it, under French law it is forbidden to use "strike breakers" - so there may be an issue for the company to deal with going forward.
Faced with the same scenario, what would you honestly do? Would you risk your job/livelihood ? Really?