I applied when I was 18 and wasn't successful, but thankfully I got in 2nd time when I was 21. The reason for the first fail is that they were unsure with the Personnel bits and felt I needed to 'get out a bit' so I took a year out and had some fun and did a little work, then went to Uni which I left after the second year of a 3 year course to start at the college. I wasn't doing any relevant course and it certainly wasn't what anyone would call a 'respectable' University, but it showed NATS what I was like. Overall I have 9 GCSE's, 2 A-Levels (Maths and Geology), and a diploma from Uni (Geology) for completing 2 years successfully although I hadn't got this when I was accepted, but wasn't far off.
I wouldn't start to second guess what NATS wants to hear or see, they will be looking for you to be as natural as possible and the interviews will highlight if you are or not. I also would avoid ever mentioning you thought "Swanwick was very dull and boring " but stating a preference for Approach/Tower will be heard if given with the right reasons. However knowing NATS whatever you say will have no bearing on where they post you so don't get hung up on a certain discipline and especially not a particular post. Look to the move away from home as an adventure with the most fabulous rewards, and if/when you get it go with a positive mind to enjoy it and get to where you want. I wasn't ever going to have a problem leaving home but met several on the course that did. We all bonded well and enjoyed Bournemouth and all its atributes to the full, and therefore I always recommend living near to the centre of town at least for the first part of the course just so you can all go out on the social in the evening to get a good bond in the group - it can certainly help get you all through the course.
I'm the sort of person that would say just apply and see what happens, if unsuccessful then go do something else for a few years and then have another go. Many controllers don't get in first time so I wouldn't let it worry you the possibility of failing first time round. If you are unsure about moving and living away then try moving away first, get a job a few towns away or go to Uni and see how you feel. I find that in NATS where ever you go you find people that you can mix with and form good friendships and that makes the move so much easier.
Its not easy and no-one here can give you a definate answer on what you need to do, but many will advise that if you get there its certainly a fantastic feeling and well worth it.