Having not performed very well during my undergrad, achieving a 2:2 from Newcastle University in Politics, I have always felt like a failure since and have always wanted to redeem myself academically.

I work currently for one of the big 4 however feel as I didn't enter through the grad route, I will never progress without further education. I am now wanting to start a Finance and accounting masters in january, ultimately for career progression and better paid opportunities. I enjoy working in an audit environment and therefore finance and accounting felt like the right masters. Despite my disappointing degree result, I have been accepted onto 2 masters to start in January however I am still undecided:

Finance and accounting management Msc - Royal Holloway
Finance and accounting Msc - Westminster

Royal holloway is the better university however the westminster course makes you exempt from 9 ACCA exams and could be the best route towards being a qualified accountant or a more finance based role whereas i'm not sure if the 'management' aspect of the Holloway course puts me off somewhat and there would be less opportunity from that masters

Any advice or anyone who has studied a similar course would be much appreciated!

I understand well how you feel. You want to prove it to yourself not to anyone else. Being biased against "managment", I would recommend the finance and accounting Master. It looks like you love the subjects more and the chance that you will do well is higher if you have the passion. Moreover managmement career is comlicated and depends on many things the least of them is education in management.
But are you going to leave your job or work parttime? Are you ready for this massive change? I am not scaring you as I did several massive changes and did not regret them but just be prepared for the big change.