In answer to the original question, if you can get into NATS do it that way. It's by far the cheapest way - and if you can fit into the NATS mindset it probably offers a better range of options for the future. My final thought is that if you end up staying with one organisation for the whole of your career, NATS is almost certainly going to be the best option financially. But there are other choices and there are plenty of people out there who followed different paths.
On the point of traffic levels during sim training, I'm fascinated to know what ICAO standard traffic levels are - I don't think I've ever seen it described anywehere! I've always assumed that the standars on any particular training course were largely defined by the customers' needs.
In reality, I suspect that the liklihood of success is determined more by natural talent, quality of OJT and trainee attitude than by what he or she can handle in a sim.