G10
1) Almost unbelievably, there is no "minimum stay" with NATS after training. I have known students to resign on the day of graduation, although it is very uncommon.
2) Not common but it does happen.
3) The medical requirements, including eyesight, are essentially the same as a Class 1 pilot's medical. In fact it IS a Class 1 medical. The only difference is where you work as an aerodrome controller only i.e. not approach/radar or area - then the minimum requirement is a Class 2.
Hope this helps.