I'm not sure of the sponsorship criteria these days of the few that exist but I would still try, what have you got to lose?
When I was still within the age criteria pre 9/11 my rather interesting array of grades at A-Level still didn't stop me from getting to the first stage of a sponsorship selection...stuffed that one up nicely though thanks to my verbal reasoning skills!
I stand to be corrected but if you can prove yourself in the application form and subsequently in the initial assesment tests your grades in the Scottish Highers will be an issue. I would certainly consider the implications and planning necessary for undertaking the course with your own funding but you aren't even 20 yet so that gives you at least 6 years of applying for sponsorships whilst getting on with your PPL and hour building etc.