Callum,
Sorry to hear you didn't make it through. I know it might say that it's not NATS policy to give feedback, but people in the past have received some, so it's definitely worth a shot - at worst, all they can say is no!
I don't know how old you are, but if you're under 25, you could try Eurocontrol if you're still keen and looking for something to do in between now and when you can reapply to NATS.