You A-level grades are pretty irrelevant
It's about the area of research and the PhD supervisor you pick and the competition within that field, both for places and funding.
My offer was conditional on getting a first, but lots of people don't have them. In fact, met a couple of people with 2:1s from Hertfordshire, but this was for Chem, which has lots of places for PhD students, and thus a broader range of backgrounds.
Once you get into the Uni a place at a college is a certainty. Whether you get the college you want is another thing, and to be honest a bit of a lottery and there i no college interview as such. I put down Emma and Catz, and got into Emma.
If you put down somewhere like Trinity (Cam), which is well internationally well known, the chances of getting into that college are much smaller than (say) Fitz. Some people choose their supervisor's college.
Another point - nothing to stop you applying to both Oxford and Cambridge (unlike undergrad)