a RG uni is helpful to make a stronger application for the top London firms, but it's not a requirement by any means. Did you get a 2.1 and what uni did you go to?
A masters makes very little difference to your application strength so I would only recommend studying for a masters if you want to for academic interest. To make yourself a stronger candidate for law firms you're much better finding practical experience in the workplace paralegaling or similar to strengthen your application.
Finally... there's much much more to commercial law beyond the magic circle - realistically at a trainee level other than it being easier to go on overseas secondments for one of your seats, your training at an MC firm compared to an SC or other large commercial/corporate practice is going to be very similar. Don't worry about specifics and just focus on making your application and CV as well rounded and strong as you can.