Within the UK, how are roles corresponding to academic grades defined? Within the United Kingdom (or perhaps just England and maybe Wales?), academic pay scales are divided over different pay grades.  Annual salaries outside London range from £16,131 in the bottom of grade 1 to £65,454 at the top of grade 10.  For example, see Oxford Iniversity, University College London, University of Reading, University of Leicester, etc.

It appears the pay scales are uniform (albeit with a London bonus), but is there any uniformity of the definition of the roles corresponding to each grade?  In one example I am familiar with, grades 6 and 7 are effectively postdocs (where grade 7 can be PI but grade 6 cannot), but this is one example for one role at one university.  It does appear grades 6 and above are usually/always considered different from grades 5 and below, perhaps due to requiring a PhD.  I wonder if any rules apply nationally.

How are the roles corresponding to academic grades defined within the United Kingdom?  In case there are differences between constituent countries, I welcome answers for all countries.

This similar question and its answers make no mention of academic grades, and therefore this does not answer my question.
As @StrongBad notes, here and in the other answer, there is a national salary spine for academics and academic support staff.

Typically (ie, universally in my limited experience), these are grouped into overlapping ‘grades’ or an equivalent term.  Thus here, grade 6 is spine-points 25–29, with three discretionary spine-points above that.  This is, I think, primarily for administrative convenience and will tend to correlate with a (loose) job description, or with the amount of ‘leadership’ and grant-getting expected; for example, you might have to be on a particular grade before you'll be allowed to be a PI.  Thus a post-doc will usually be grade 7, a lecturer or ‘senior’ post-doc grade 7 or 8, a reader grade 9 and, as noted above, professors are on a notional grade 10 which isn't on the spine.  People will tend to automatically rise one spine-point per year, until they reach the top of the grade.

So the answer to your question is that although the details will doubtless vary between universities, the grades and their consequences/expectations are probably pretty common in UK academia, without there being any specific agreement or constraint.

If I look at some job adverts, I see only salary ranges mentioned, but if I look at detailed job descriptions, they're articulated in terms of grades, and I see phrases like ‘The salary will be on the Research and Teaching Grade, level 7/8/9, £33,574 - £37,768 / £41,255 - £47,801 / £49,230 - £55,389 per annum’