Breaking into Finance with bad A Levels

It might be challenging to go in through the front door with the graduate recruitment process if your target organisations will be taking your A-Level results into consideration. However, the Russel Group status is likely to be helpful. So you might consider to begin expanding out the other segments of your CV and profile with additional features that are going to stand out. Work experience. Extracurricular activities that evidence your capabilities beyond academia. Work shadow experience. Additional training, certifications, and so on.
Don't forget the value of networking - now and in the future - to try and connect with individuals that you might know (or have something in common with) who work at the target banks. For example, can you see anyone on LinkedIn working there who also went to your University? Can you reach out to some and see if you can introduce yourself, buy them a coffee, ask if they can suggest advice for you to break into the industry?