I recently went through the whole process, I managed to get to Swanwick, West drayton, but also non NATS units Plymouth and Exeter. Its like the post above says, I just rang around. I was very polite, and in some cases where they were not willing to help over the phone, I wrote a formal letter requesting permission to look around. I managed to spend 6 hours in West drayton, speaking to a whole host of ATCO who were very helpful. I did tell the interviewer about all my visits, I passed and who knows how much my visits had to do with the overall outcome?