As to the percentage of people that are selected I had my phase 2/3 combined assesment day a few months back.
The assessor said that large numbers of people send in the application form online but only around 20% of those that pass this first stage bother to then go on and pay the £190 and book an assessment day.
This is probably a large factor in the 2% pass rate figure people are suggesting, and its more likely that 2% (or more) of total online applicants go on to receive an offer for the "Wings" cadet course.