Well, I imagine in the first instance, there is a sickness reporting policy at BA (but I do not know the details, as I no longer work there). This will lay down the procedure for sickness reporting. I would imagine that if it follows the general accepted procedures (there is no specific law for this), then there will be a self certification procedure upto 7 days.
Is there a separate policy for sickness when in dispute? Is there a specific cabin crew policy? What happens to other employees who are sick when a strike is on? What happens to non union members, who cannot strike, who are sick? As one of the mods has stated, you may have to pay for a fit note too. If you have a cold, that may have passed by the time you get an appointment. A common cold has greater implications if you fly, than if you do not.