On the original question, if you will be training as a controller with NATS then I think your job will be as safe as it's possible to be these days. There's already a shortage of controllers, there are always retirements (whether people refer to it as a bulge or anything else) and despite plans to automate the system and do away with the humans I doubt that we'll see a significant reduction in the number of qualified controllers in the span of your career.
The loss of some airlines or routes simply opens opportunities for other operators!
If it's a job you really want to do, throw yourself into the training and get qualified.