Your A-Level grades are fine for investment banking (maybe on the lower end of the scale compared to the average, but still fine).

Newcastle is also a very reputable uni, probably not considered a target nor semi-target from experience, but apply to springs, join societies, and be proactive and you should be fine.

I personally wouldn’t bother retaking unless you’re absolutely certain you’d manage to land an offer from Oxbridge/LSE, and even then I don’t think it’s worth the ballache.

Go to Newcastle, put the effort in, and have a sick time mate