You need to know both though, and IIRC the majority of the paper focus on ATC related areas and not aircraft specifications.
It may also be relevant to know since that will give you an indication of where you can expect to find each aircraft and what characteristics that aircraft type has. It may be good to know that a 777 will cath up with a DH8A. Or that you most likely can not get that 767 to fly 120 knots on final approach. And the amount of pax carried is an indication of the size of the aircraft, which is a key indicator to things such as approach speeds, ability to slow down or go down quickly and wake turbulence generated.
Not to mention that if you get through you will need to learn all this anyway, I don't really see the problem with doing it now? It also shows NATS that you have an intrest, if you're not even willing to learn the basics about a few aircraft then how will they know you are ready to put in the time needed to make it through the college?