They can't offer to work rest days as GTR has banned them from doing it. And going sick now requires a GP's note, which means anybody off sick needs to be off long enough to obtain said note; if they return to work after a few days they won't get paid. So the odd cheeky 'sick day' is not an option, unless people are going to string it out for a week or more.
All of the above is irrelevant however, they're cancelling trains even when the crew are sitting in a depot ready to go. They don't care, they are doing it intentionally. Partly in a cynical attempt to persuade the public they'd be better off without Guards, and partly becausr the DfT is their master and has pretty much told them to do whatever the hell they like without consequence.