NATS seems to be very inconsistent with its advice about ATC visits. Everyone I have encountered outside of the HR Dept has been very helpful with respect to arranging a visit. Still, it wasn't to be for me as I crashed and burned in the final interview.
Contact Terry Ward at Swanwick and he's the guy who can help. Think the visit took about 90 minutes. Switchboard number for NATS is 01489 572288.
Good luck!