I'm 19 and I passed Stage 2. If you have minimum requirements they will accept you, just make sure you look professional (take special attention to your hair, as if you have long hair with curls it won't look as professional as if you had shorter hair - my case, so I had to adopt another hairstyle just for NATS
). If you really want it, the journey will be nothing (you can revise the booklet), in my case I'm from Portugal and I passed 18 hours travelling just to do the ATC Knowledge Test (already taken and passed FEAST and DART at Eurocontrol).
Tips for the day: calm down, try to make friends and plan ahead (if they asked you to be there at 8:00, try to get there at 7:20 to avoid possible delays due to traffic or.just use the good old public transport - like me
).
The test will be fairly simple, just hammer down the booklet (they wont ask you things like "What is jetfuel maid of").
If any of you are thinking about staying overnight at Southampton, I recommends the ibis Budget as it is fairly cheap, clean, confortable and is really close to the rail station!