Security checks can take a while, but they will allow you to start your training conditionally until your checks come back. I think mine took about 6 months, and I'd been at the college for just under that! Yes, there is a CTC involved. It's the medical that's the important one; as long as it doesn't throw up anything that could cause problems you'll be allowed to start, but you can't start without a valid medical certificate. As has been said, the time between and acceptance is dependent on when the next course is, and how many spaces they have left to fill. They will probably ask you when's the earliest you can start, and it's best to be honest because they will take you up on it if they can!