Retaking A Levels or doing GMAT to get over hiring barriers?

Currently working at a small consulting firm as an Associate Consultant. Have worked on a number of strategy projects over the last two years e.g., market sizings, full scale go-to-market assessments, and have been getting consistently strong feedback from senior leadership, usually taking on a lot more responsibility than others at my level. Salary is terrible given the workload and the firm doesn't want to invest in training e.g., data analysis, modelling, so I've been looking at moving firms but I'm concerned that my education is holding me back. 

Took Maths, Economics, and History at A Level and scraped by with a C and two Ds after leaving schools halfway through year 12. Realised I wasn't getting into a respectable university with these results so pivoted to do a creative degree at a recognised university where I received a first before ending up at my current company. I've taken myself a lot more seriously over the last two years and I'm grateful to have ended up where I am, but I feel like my past is going to hold me back from making a meaningful career change. 

For example, I would love to do a Masters in Finance to continue building my credentials and open the door to finance-related roles but this feels impossible with my non-traditional background. Even when applying for roles, firms usually want to know what your A Level results are. I recently applied to Bain as an experienced Associate Consultant hire but quickly felt disheartened when they asked for my results. 

I really feel very trapped and I'm not sure what the best course of action would be. Retake my A Levels or practise for a high score on the GMAT? Do an accredited course like the CFA or some modelling courses in my free time? Any advice would be greatly appreciated.