This misunderstanding is quite often perpetuated around Facebook groups; I am slightly surprised to see it here.

The correct statement is that the UK CAA says airlines should (my emphasis) seat under-12s within one row of an adult in the same party. That would include, for example, child in 29B and adult in 30K.

Now, on a day to day basis as mentioned elsewhere BA is very good at going above the minimum, though it isn't an absolute entitlement.