As an addition to scouts/forces I would suggest Teaching English as a Foreign Language, tefl.com. Particularly if you can get a placement to somewhere remote – this would give you great teaching and communication skills and a year away is fantastic experience.
I think you are being unrealistic in your timescales for your objectives but I think 18 is a great time to start! Couple of thoughts for qualifications on this thread:
http://www.bushcraftuk.com/forum/showthread.php?t=17914
If I was setting up a company in the bushcraft field – not with me as an instructor as I know very little - I would be finding out the below just for starters.
Qualifications: What is - required, best practice, commercially advantageous
Experience: quantity, diversity, teaching, bushcraft, other
Specialism: can I – demonstrate, compete, be knowledgeable in one subject above others.
Legal & Regulatory: HSE, CRB, Business
Cost: start up, ongoing
Competition: what works, gets good reviews, adds value.
Branding: name, website, style
What you can do is answer all the above and decide whether you want to commit the next couple of years to it. You can work instantly on the branding – lots of posts are telling you that you have got it wrong at the moment - create a professional image. Got to say just looked at Jon’s website and it’s an excellent example of demonstrating skills. I suspect that the best and cheapest way to cover much of the experience and qualifications area is working with someone who already does it.
Good luck, Matt