Freehills-
Those attorneys/docotrs have specialties. NHS has many doctors, but they are not all General Practice doctors. While I appreciate your numbers, I don't believe them to be an accurate reflection of our profession.
We have 3 ranks within our 3,000: CN, FO, SO. Four if you include check and training, that's it. Attorneys/doctors have many more catagories within their fields, which provides multiple iterations.
The first firm you listed practices 13 different fields of law. Within each of those fields are many different aeas that requires specialized attention. We just don't have that structure.