ThomsonFly as with any airline (even ones who sponsor!), especially in the times we live in cannot possibly gurantee you a job before you have even started training.
What they can do is see a trend for an increasing requirement for pilots and try to do something about their requirement.
With ThomsonFly they are expanding their low-cost arm and (I believe) looking to increase their market share in the long-haul market whilst having a high retirement rate over the next few years. Therefore if things continue to go to plan they will need more pilots and those on the scheme will be their latest 737 or 757 drivers!
If for whatever reason there isnīt any jobs at the end of your training you would be free to go and find a job elsewhere. I also think that you would be in a good position in finding a new job as the airlines will know that you have already been through a tough selection process.
Anything can happen in this industry with reagrds to spare seats in the pointy bit and the only person who can decide what to do about it is yourself. But as StudentInDebt says keep your eyes wide open and make the best educated decision for yourself.
Sammy