UK legislation unfortunately does not allow UK based pilots to strike or refuse a duty which covers the French strikes. To do so would expose the individual and BALPA to legal action. This is from the union. If we could support them we would. Everyone calling in sick for these duties could be seen as wild cat action. No-one likes it but if you are rostered to go to France, you go.
However this will all be at a cost to the company. Therefore the company have a tangible cost of industrial action carried out by the French pilots. I imagine it is causing quite a bit of disruption.
I think before some people throw around comments they need to investigate the legal landscape surrounding industrial action in the UK. I doubt they will do this though as they seem to be unable to even read and comprehend posts on here properly let alone something a bit more heavy.