It will be interesting to see if WW does start trying to sack strikers after 12 weeks. My hunch is that he won't, for two reasons.
First of all, I think BA wants to keep the moral high ground in the minds of the travelling public. Whatever side of the fence you are on there is no doubt that, currently, the world outside thinks strikers are mad and that BA is right to take them on. I think WW will want to keep it that way and won't want to be seen to act unfairly.
Second, the law is not 100% clear on the 12 weeks issue. I guess this perhaps isn't a surprise but it was a law designed in the days of all out walk outs. The question of whether it means 12 weeks from the date of the first strike or 12 weeks of strike action has never been answered and, so far as I understand it, is not clear in law. Of course, lawyers will all have a view and I don't know what BA's lawyers have been telling WW.
How will he deal with the strikers then? We know how because we know what he is doing now: ramping up the volunteer programme and perhaps recruiting more temps. Eventually strikers will be sat at home, on basic pay, doing nothing, prevented by their BA contract from getting other paid work (if they do, there'll be an excuse to sack them), unable to afford the mortgage or the finance payments on the SLK. WW might eventually offer them new fleet contracts to sign and 90 days notice. Or say their jobs are now redundant and they can take a redeployment opportunity elsewhere - perhaps cleaning the loos (I know it'd be a lot to pay a loo cleaner but think of the poetic justice). Who knows. Eventually they'll become irrelevant. WW might even say after a while, "We have this great new one team approach on our flights, all the cabin crew who are coming to work are fully participating and being very professional, they're all having a great time at work outside the shadow of the union militants; sadly, we think there might be serious safety issues around all these miserable strikers coming back to work so with great regret we've got to let them go. The CAA are fully behind this move." All very sad and, like many have said, all very unnecessary.