As for one of the regional units. Who would I need to speak to at NATS? would it be HR? Of the few people I did speak to, none mentioned a 'visit co-ordinator' at another unit. Do most units have these?
Most small units probably won't have a designated co-ordinator. The easiest method is to look in the AIP for the ATC number listed at each airport and give it a call. You could also try giving the airport main number a call and see if the switchboard would put you through.
But the numbers in the AIP are published for public reading so there is no harm in ringing them. Applies to NATS and non-NATS airfields. Not sure if NATS HR would give you contacts or not.