The reality is a degree is somewhat useful even after you're qualified a accountant. If you look at manager level job adverts, some percentage of them expected degree educated candidates.
But on the other hand AAT followed by CIMA/ACCA is a solid route into accountancy and once you have the CIMA/ACCA you could be on a level playing field with degree educated candidates, depending on the employer to a degree.
Moving into investment banking is a bit of a far shot like you suggest. From my experience, at your stage in your career many people wish to go into investment banking, whereas a few years down the line many people realise there isn't any point and the sacrifice to work/life balance is not worth it. So I would have a think about whether this is a sensible & realistic ambition.
So if I was you I would look at entry level positions either with AAT study or do it in your own time.