A top 2.1 from a top 20RG is more than sufficient to work at the large London firms - most of their trainees and junior solicitors will *not* have attended Oxbridge & an Oxbridge degree is, generally speaking, only a slight advantage in the early stages of the application process. Once making the cut to the first round of interviews lots of other factors are far more important - how well you interview, evidencing a commitment to law, legal work experience & vac schemes etc...