Hi Tilly-Elizabeth. You will absolutely need a degree, preferably in Marketing, and essentially work experience through summer internships or a placement year. Marketing requires numerical, presentation and creative skills which can all be learnt at a good university with help from live work experience. Also, whether rightly or wrongly, the best employers value higher education. With the sheer amount of students in the UK with a degree in relevant subjects, not having one would go against you, just as lack of experience would.
I really hope this helps you make an informed decision. I know some people will disagree with what I've said, but I've been there and done it - it's the brutal truth. In the current market place, it's common for those without a degree experience the 'glass ceiling' in the best jobs in business. Of course there are the few exceptional employees who we don't here about, but to give yourself the best opportunity at achieving what you want from your career, higher education would be a good decision.