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I've moved this thread to our armed forces careers section where hopefully you'll get some useful answers.
To attend Welbeck you have to go through the selection process for whichever branch of the military you want to join, basically the same as people applying direct to be an officer in the military. If you are accepted you are expected to keep up with your studies, meet fitness standards and take part in sports, extra-curricular activities and Combined Cadet Force, including going on exercises. You then go on to study at degree level and, if you graduate, go on to officer training.
Does it increase your chances? It's certainly giving you a good foundation, and shows commitment as you are having to undergo interviews and physical tests at 16. However, plenty of people join the military as officers in technical jobs without having been to Welbeck. In either case, it pretty much comes down to being prepared for selection and putting in the hard work during the learning part.
Welbeck Defence Sixth Form College is for Engineer Officer applications only as it's the only role the RAF puts through there. If there's another role in the RAF you're interested in then you would need to gain the required qualifications prior to application. I'm not sure about the other services.