When did this dubious practise of handing over money for a sicknote start, and where
As you can self certify for the first 7 days, most doctors will not issue one for that period. They class the request for one for those days as a private note & charge for it.
but why should you have to spend some money to prove you are sick
I would guess (well am fairly certain) that at BA you do not normally have to, it seems to be an attempted counter to the going sick instead of striking ploy(?).
I'm sure BA know roughly how many staff would be ill on a daily basis so if that number peaked on strike days (despite BA warning staff in advance) they would see it as possible avoidance tactics