All I can say is that there is no certain answer. You may or may not be successful in getting a job weather it be airline or instructor. You just can't tell. It's impossible to see what the industry will be doing in 23 years time. It's a chance you're going to have to take if you're certain about living both your dreams.
If you've got 20K at the age of 17 you're in a much better financial position than most wannabes and I'm sure you'll use that to your advantage!
Right now I can honestly say I want the RAF more than flying
- if thats how you feel then I'd suggest you concentrate on giving the RAF your all. Sometimes it's best to concentrate on the things that inspire you the most. Doesn't mean to say you can't continue your flying whilst you're working.
S88