Do people applying for NATS interviews actually have to know everything that the various courses are intended to teach? I wonder sometimes just what is expected. The selection tests should be to determine suitability not to see whether the person already knows it all?
If asked about separation rules then, in my view, it ought to be enough to be able to say that there are vertical and horizontal separation minima.
Vertical is basically 1000ft but more in some circumstances.
Horizontal separation (i.e. for aircraft at the same level) can be determined by "procedural" methods or by radar. Procedural methods are based on aircraft flying on different parallel or diverging tracks or, for aircraft on the same track, by time. Radar separation is basically 5 nautical miles but may in some circumstances be 3nm.