Try visiting a control tower or area control centre.
I passed NATS selection back in November and found that such visits were exceedingly helpful in getting an idea about what the job is really like and whether its something I could enjoy doing with my life. To be honest, I found the job to be quite different to how I'd imagined it, but for me those differences were all good ones which made me want it even more.
Sometimes visits can be a little hard to arrange, but if you are able to get hold of staff at a particular tower or facility they are usually happy to have you for a few hours.