SebbyK,
I've just been arranging a visit to Southampton control tower. There doesn't seem to be any centralised process for it, but if you can get through to the staff at the tower in question I found them to be really helpful. In the end I just phoned the airport help desk, got the number for the admin office in the control tower, and left a message for the watch manager. They got back to me that day and arranged a visit for the following week (though I had to cancel because of swine flu
.. but I'm just waiting for a call today to confirm they are ok for me to come in this week). If you explain that you are sitting the selection tests they are quite understanding and happy to help.