Scooby check with the union about your days off but like everythinh else they will not know the answers...hence the position your in now.
With regards to sickness...it is up to Virgin if they want to ask for a Doctors certificate after however many days they want. As lets face it...most crew are going to call in sick...eventhough these same crew said yes to the strike. Then a interview with a cabin crew manager i can't see many crew holding up under that pressure.
Just be honest and call up and say your striking if thats what you choose.