Within the UK, how are roles corresponding to academic grades defined?
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’