I'm sure all carriers would like to operate as many flights as they possibly could. If the BAA airports has a planned closure then the operations department has control over Captain instructing where to divert to.
If Heathrow and the other 6 airports are closed due to strike then i'm led to believe it will be not possible to have a slot for them and therefore an alternate flight plan will ned to be filed with an alternate destination.
It will not be possible for the airport to operate as there will not be sagnificant fire coverage for aircraft to land, plus no security personel.
Apparently Zoom, BA so far are looking at CWL if the strike does materialise.
Bearing in mind that Heathrow, Gatwick, Stanstead, Southampton, Glasgow, Edinburgh, Aberdeen will all be out, this will be a massive blow to the UK airline infurstucture and the knock on effects will last for at least a week.