Now, I believe that this clause refers to the 24 hour operation, and that you have to work shifts that may include any hours of the day, any day of the year. But it doesn't give the employer the right to tell you "you are staying on" after you finish your shift, or telling you "tomorrow you come in the morning instead of the afternoon". Not all employees are free 19 year olds without a care in the world. I have my life outside my job, kids to look after and the need of an agreed shift time in advance. I would fight any attempt to get from me this kind of flexibility. I work to live, I don't live to work.
Scaremongering. Every company I have ever worked for has this clause, and it simply enables the worker to go above and beyond in rare events by working in another area.
It doesn't mean that the firm would ask you to tear up your schedule (have you worked for an airline other than BA - clearly not recently...) - what it means in practice is that you instead of flying that afternoon, you might be operating checkin.
Boo hoo - big deal.
As I've said before, you are no longer in the world of 1x weekly cargo operation on the other side of the world, where lone 19-y/o stewardesses would be abandoned on their own awaiting the next crew to pass through.