Mine isn't opinion. It's a fact how law firms recruit. I've spoken to people in HR and lawyers. Been to presentations. Done the applications. They want a minimum of AAB, and a 2:1 degree. Most applicants have AAA at this point. Even that isn't cast in stone; I know someone with ABB who got a training contract with an MC firm. It's illogical to discriminate based off university alone. Someone who got AAA at KCL isn't better on those facts than someone better at Exeter, or Newcastle, or Dundee with AAA. Employers/HR aren't as stupid as you think they are.
You are suggesting that there is a tier system which employers use to filter out graduates. This just isn't true. There is no secret chart. You are American. You have not spoken, I presume, to HR in this country at major law firms. I don't even know if you've been to the country before. You may have read threads, but I'm telling you what happens with most employers all round the country. Because your suggestion is illogical and stupid.
As far as Hogan Lovells goes, it says these are 'some of their trainees', not all. I'm sure there are trainees, associates and partners who have gone to QM. I don't need to look it up. I know there will be. But even if they didn't it doesn't make a difference; it depends on how many people in each year want to go into law, who they are, where they apply, how many applications they make...people in my graduating year ended up at MC firms, US firms, other top law firms. Many did masters with the intention of going to the bar or law firms. Lots of people didn't want to do law at all. The same problem goes for the bar statistic. You'll see great unis like Warwick aren't recorded. It's a snapshot from a particular year; some might be taking masters. Remember also 50% of a law firm's recruitment comes from non-law students, so it also depends on whether students in other departments in the uni want to go into law too.