This is indeed spelled out by "Knowles on Local Authority Meetings" - a consequence of the clause that you quoted includes "Nor may any individual council member vote by proxy: they must be present at a meeting if they wish their vote to count". Although you're quite right that the sentence is often included in standing orders, it is actually part of the legislation, so applies whether or not it is included in standing orders. LGA 1972 Sch 12 39 (1)