1. No law firm or set of barristers' chambers cares about any university ranking table.
2. See above.
3. Er, that's it.
I have been considering applications for mini-pupillages and pupillages at two sets of barristers' chambers since the 1990s. One of those chambers is a magic circle chambers. I have also been a partner in an international law firm and made hiring decisions at that firm. I am a consultant to a law firm and assist in hiring decisions there. I teach part time at UCL and have also taught at QMUL, Reading, and at Trinity and University Colleges, Dublin.
At my current chambers and at my previous chambers, applications for mini-pupillages and pupillages are considered without disclosure of the university from which an applicant graduated.
When considering lateral hires, experience in practice is the key factor, with academic record a background factor.
I have asked around amongst colleagues at other sets of chambers and at law firms. Nobody has told me that their chambers or firm considers any university ranking when making any recruitment decision.
There is good reason for this. First, we are interested in individual talent, not in reputation by association. Secondly, the rankings are compiled on journalistic bases and aren't reliable. They are based on small sample opinions, they use varying measures chosen at random, and they sometimes present quantitative information as though it were qualitative (for example: "research quality").
Candidates from some universities tend to do better than candidates from other universities because of the resources available at the more competitive universities, and the quality of the education which those universities provide. When you consider that many colleges at Oxford and Cambridge have endowments larger than whole universities, you can see the resource disparity. Oxford, for example, has over one hundred libraries (that's one library for every two hundred and sixty students).
The most competitive universities can pick and choose from the most academically successful sixth formers, and can also pick and choose from the best academics when the universities recruit academic staff.
So, yes, you should work super hard and try to get into Oxford, Cambridge, UCL, LSE, KCL, QMUL, Bristol, Durham, Warwick, Nottingham, and so on, but please don't obsess about rankings. The law firms and sets of chambers you aspire to join don't care about rankings.
TL/DR? Law firms and chambers DON'T CARE ABOUT UNIVERSITY RANKINGS.
PS: Russell Group (1) has two ls; and (2) is a group of universities which have a loose association, mainly for marketing. The Russell Group universities are good universities, but non-membership of the Russell Group is not a sign of being no good.