Right, the problem with I find with trying to give advice with things like this are how one direction that works for one could be a disaster for someone else. Here's what I did anyway...
I went to university. I didn't know what I wanted to do when I left school and university seemed the easy choice. I did 4 years and walked away with an Honours Degree in Computer Science. There was no jobs out there for me when I left. None. They all wanted experience and everyone mentions that catch 22, how can you get experience if no one wants to give you a chance in the first place? I went home and got a job in a call centre just to keep me going. Within a month I was moved to a team lead role and I stayed there for 1 and half years. I then got an interview to for a programming job in a large bank in Scotland. I got that and I've been working there for just over 2 years (and enjoying it).
University worked out for me because it did give me that degree that helped me get an interview but it gave me a lot more. I was incredibly shy at 18 and had very little confidence. Uni was where I felt that all change and I'm feeling the benefits of that experience more now than the 4 years of knowledge they tried to stuff down my throat. I don't think I would have as good a job just now had I left school at 18 and did a trade (most people I know go in to a trade). But that's just me. I hated actually going to uni, I skived and I'm surprised I passed but it helped me so much.
I don't envy you. I hated that decision and in hindsight, I think I made the right decision. Scotland helps you take that uni risk because, unlike England, it doesn't charge you ridiculous fees. You can take a chance on it, and most do. I know a lot who have tried and failed in the first year and it's not a huge issue. They weren't interested so they wasted a year (and may have learned some life lessons and not ended up with a ridiculous fee at the end).
I guess my point is, because I think I'm rambling a bit, is that my story is pretty irrelevant to you and one of the hardest choices you'll ever have to make. If you don't want to go to uni, don't. If you want to go in to marketing, try and find an apprenticeship. Look for open college courses. E-mail companies and ask them what they look for in candidates and any recommendations they might have for you. You seemed to have a good attitude. I know plenty of people who didn't even want to face up to this, or went to uni because their parents wanted them too, or where quite happy to sponge of their parents. Your attitude puts you a step ahead of a lot of people your age.
Good luck with what you decide to do, loads of people on here will have better advice than me.