I would suggest you visit as many as possible. When I told my interviewer I had visited 4, it created a positive responce.
I managed to get a visit to Swanwick, West drayton and two small airports not run by NATS.
I am in Plymouth, so obviously it cost me money to make those visits, but who knows it may have swayed the interview in my favour, as I was succesful.
GOOD LUCK.
I have know idea when I will be on course. Reading some of the negative posts in these forums it will be never. Maybe we will be on the same course??