Lots of spoilers, so read at your own risk.
They tested the kids on almost everything, from mental fortitude and physical fitness to pain tolerance. But the "watchers" forget one very important thing: loyalty. Realistically speaking, why the ef would those kids who were practically kidnapped suddenly want to defend the UK from terrorist attacks? Sure, there might be a few, but for the other 50-100 kids, who, by the way, came from all over the world (as Woody mentioned), including the US, Afghanistan, South America, Australia, etc. Again, why the hell would they fight for England?
Their system of recruitment is just plain stupid.
When the MC is about to be booted out, miracle of miracles, a situation occurs that gives him a second chance. It seems there is a mission only they can do. Huh? Why? The reason they are recruiting new special forces cadets is that the last batch is getting older. They are getting older but still young. The fact that they have survived until now means they are better suited to deal with the situation. But nope, the MC's team must be the one to deal with it because ... what? Destiny?
Look, the premise of the book is good, but the many plot holes because of the bad plot outline are just too big to force you out of the immersion.