at the airline i work for a few years back we were required to provide a doctors cert but the union challenged it and it was dropped, now we get a pre christmas notice asking us to give 'plenty' of notice to crewing if possible and pointing out that caual sickness will upset others xmas plans! as far as i undertand under UK law you only need to provide a Dr's cert if you are sick for 7 days or more as less than 7 days only requires self certification, however i'm sure 'some' employers do use this as a threat tactic, if your sick your unfit to fly, if its casual you could end up messing up other peoples plans!!