For anyone who still thinks this strike is actually about Open Skies (Slim20 take note), please re-read this thread and the other related threads.
The strike is about amendments to Schedule K of the pilot's Operating Agreement. It is NOT about Open Skies per se.
Schedule K is a scope clause which protects career opportunities for BA pilots, specifically that BA mainline pilots must fly any BA mainline aircraft of more than 100 seats out of the UK and any BA aircraft out of London. The dispute is about whether or not this agreement should be extended to cover European operation.
Open Skies has made the news as this is what triggered the current disagreement but the strike is about Schedule K. The law in the UK does not allow you to strike for something which might happen which is why it is not about Open Skies and is about amendments to Schedule K.