Here's my background.

Went to local comp and finished with good grades (GCSE:9A*2A, ALevel: 4A)

Having had no advice on picking Universities (and not really knowing what was out there) I opted for marketing at a not-so-great Uni (top 60 in UK)

Course was easy, and I began to consider whether I should have pursued an academic course at a top Uni, however I stuck it out and graduated with a first

I was invited back to begin my PhD a couple of months after graduating from my Bachelors degree (fully funded by Uni)

I really enjoy my PhD subject (more social sciences than Marketing really), am progressing well and have published despite only being 18 months in.

I worry that I will never be a top academic because of my education. I draw from sociology (and anthropology) literature in my research, and considered whether I could move disciplines after finishing my PhD, but fear that I don't have the right training to even be considered.

It seems to late to anything about it- I could have potentially tried to get onto a Masters at a better Uni using my A Level grades to illustrate that I do have potential, but after PhD none of my previous education will count. I feel like I made the wrong decision at a young age, and will be forever paying the price :(

Does anyone have advice?

If you think your previous education will hold you back (and I don't necessarily agree!) then there are a lot of things you can do now to really make you stand out. You have made a good start as you say already have a publication; you should focus on getting as many first author papers published as you can before you finish your PhD. Collaborate with top experts in your field wherever possible. Get your work seen at as many conferences as is humanly possibly, present posters, give oral presentations in the UK and abroad. Get some teaching under your belt. Is possible at your uni to supervise Masters students?
Basically, just do everything you can to make you look like an outstanding, hard working student and this will override the fact that you don't have a Masters or a degree from a top uni.

You will struggle to move discipline to sociology / anthropology, because there are far fewer jobs and more PhD students trying to get them than in marketing. However, you've probably got a better chance of getting an academic job in a business school than in any other discipline I know, so maybe look on the bright side about that! There's nothing to stop you collaborating with sociologists in the future even if you are based in a business school.
Smoobles' advice is spot on - I'd also suggest (as a social scientist) that you take as many research methods courses as you can during your PhD - look at the Essex summer schools for example if your own university doesn't offer much. I'm guessing from your description of your university's rank that it might not be an ESRC doctoral training centre. If not, look at ESRC DTCs that exist, see what training they give their PhD students in your field and see how you can replicate those skills. Methods (especially quant but also good qualitative skills) are becoming so important in social science that a decent grasp can really help to open up opportunities on inter-disciplinary projects.

From my limited experience in academia, there are MANY MANY ways to go to the same place. So your background is irrelevant to the research you are doing.It is absolutely possible that architects, medical doctors, microbiologists, chemists, building engineers, educationalists, epidemiologists and a few other disciplines might be working in the same research area. True story.

What do you mean when you say top academic? The top academics in our department don't do any research at all. They are under constant pressure to bring the money in, so they spend their days writing grant proposals, managing people and managing money. No thanks, I'd rather be a research assistant forever.