Kev, emu
Great idea to visit an ATC unit pre- interview. If nothing else it shows a bit of initiative and motivation. Spend as long as you can on site. Depending on how much you you already know abut the business, there's a lot of info to try and absorb. I don't work for NATS but if you've got any general ATC questions feel free to e-mail me.
Best of luck to you both.