Hey
I work in marketing (here at TSR) and have employed a fair few people. I've always recruited at graduate level, but there are people who come into the industry at a lower level.
I think the challenges you'll face are probably in two areas.
Firstly you may find yourself being channeled into a specialism. If I took on a non-grad it would be to do something very specific. At TSR it might be to work on the ad serving team who process all the ads and use all the smart ad tech we have. Whereas when we appointed generalist marketing execs I would look for a graduate. This specialism is a blessing and a curse. You could quickly become quite specialist in a specific area, but might find it harder to transition to something else.
The second problem kind of follows on from the first. A lot of the more generalist roles are advertised at grad level, so if you wanted to move into a graduate level job (which would be more varied) you might have some tricky conversations with recruiters. It's not to say that you couldnt or wouldnt be successful but it you might need to fight your corner robustly.
One thing that could make your life easier is to get a very early start on your CIM qualifications. Finding an employer who will support you through Chartered Inst of Marketing exams, which you would be well advised to take regardless of if you go to Uni or not, could be invaluable. that way when you're at second jobber stage the employer would be faced with a difficult question - "I value a degree but do I want a grad fresh from uni, or someone with meaningful experience and a professional qualification?"
I've been doing this for ages now, happy to answer any Q's :-)