You may have seen today's Higher, which estimated the number of AAB students in certain institutions by looking at people with 450+ UCAS tariff points. Obviously 450 points is significantly more than you would get from 3 A-levels at AAB, but very highly qualified young people will tend to have other tariff-bearing qualifications such as Grade 8 in one or more instruments, or a British Horse Society certificate.
I have spoken to colleagues at BIS who confirm that the exact methodology will be for HEFCE to consult on, but their policy intention in writing 'the equivalent of AAB or above at A-Level' in para 4.19 of the White Paper was not to refer to the UCAS tariff. Presumably they wanted to leave some flexibility to include qualifications which elite institutions do recognise (such as International Baccalaureate).
Then, literally whilst I was writing this post, HEFCE published their consultation and set out the rules in Annex C. UCAS tariff has nothing to do with it. They have also promised institutional-level modelling of the impact, which will be great to see.
The implication of using a very restricted set of AAB equivalences, as HEFCE propose to do, is that there will be even less impact outside the elite from this measure. I'll wait until HEFCE publish their model before I write more, but I will note that HEFCE have expressed concern about subject provision and Equality & Diversity impacts from this policy. I think they are right to fear both.