MANAGP,
The ATCO Union is fighting hard to get the restrictions lifted! I think I'm right in saying that ATCOs are on the DoT approved list but it is up to each company whether or not they allow flight deck visits. Also I believe there is an issue regarding that the ATCO must have a proper ticket for a seat down the back and who pays the departure tax?
We are also trying to make Fam Flights part of the Unit Training Plan for Ab Initio trainees as they would have never been in a cockpit. Unfortunately at the moment, NATS won't give Time in Lieu for Fam Flights to valid ATCO's as they have to be conducted in our time off. They can't roster them as part of our duty time as we are short staffed and (probably quite rightly) won't take the service delivery hit by putting flow on for ATCO Fam Flights.
As you can see, there are hurdles to over-come from both sides, but I think it is generally agreed by all that Fam Flights are a necessity and it is up to pilots and ATCOs to push their respective companies to set up a proper scheme. Of course, Pilots are always welcome to visit the Towers and Centres as well.