Chances that you will make it are very very thin. I've had a loooooooong talk with a retired Major in the UK on my visit there(
http://www.bicton.ac.uk/news/bicton-...ceives-his-mbe, this guy, hes fusking awesome), he said that to join the army, nevermind the marines, you have to have good school results ( in subjects like English, Science , Maths) , you have to be very fit, and you need prior training/preparation. Plus UK citizens take priority over commonwealth applicants.
I also spent a day in a Royal Marine Commando base at Lympstone with him , as well as in the Bicton College which is like 10 mins away from the base. They had a SAS guy there teaching hand to hand combat to the okes who are busy with their military service course. To get to the point - the training is very hard, that is if you pass the medical/physical tests, as well as aptitude test. The Royal Marines are part of the Navy , meaning you have to be an excellent swimmer. Floating doesn't count.
I was also swept up by the hype they create on their website - once you see the real thing you start having second thoughts.
I suggest you go for a preparation course before joining, something like the course at Bicton College. Otherwise I hate to break it to you - your chances are very slim. You can always try out for the Army 1st and then the Marines.
If you are however dead firm on joining - watch out for the interview questions. If you give the wrong reasons for wanting to join it's gg. I still have all the exercise and aptitude papers lying around, if you want to get them or smthing.
I get okes everyday asking me on how to join the Royal Marines(yes marines only, because of the movies they watched and because it sounds cool) and I ask them why they want to join, and they either don't have an answer or say something stupid.
...never talk about that. Military isn't there to kill or get you killed, it's a job, just like any other high risk job out there.