Jooels,
All airlines have standby crew. They also require us to eat at different times and different locations from other crew members. This is to prevent the possibility that the entire crew goes down, allowing operations to pull from the standby list.
Thomas Cook is no exception to this.
However, there are certainly situations where even using all the procedures, bad things happen.
So, perhaps they all ate at different restaurants but all got sick. Then they call the people on standby, but its the middle of winter and they have a head cold. They call someone else but can't make it work because of duty time issues.
Your last sentence makes it seem like you think the airline simply cancelled the flight without looking at all the contingencies. You forget that they would have been looking at your flight for possibly hours before you even showed up at the airport and that cancellation was the only option.
Perhaps you should thank the airline for making the safest decision possible rather than pushing a crew who is unwell into going flying. Imagine the reaction you would have if a sick crew member made a mistake that actually caused you injury...I'd love to know if customers in your industry can come back and get money from you for making safe decisions.