Will my A levels matter for post-graduate study?

So I managed to get an offer from a Russel group university and they asked for AAB for my course, which is what I'm probably going to get, but thing is I would love to go to Cambridge for post-graduate study and AAB is sub-par for Cambridge. So my question is if I achieve these grades will it matter later on and lessen my chances of getting into Cambridge or Oxford or will these universities not even see my a level grades since I already have a degree from a good university?