tiggy - your post reads as very reasonable.
Most parents here and in RL will agree, and not see any cause to enhance childrens' knowledge at home.
But if parents got to see their childs papers, if parents were given full knowledge of how many points were needed for each level, I truly feel that they would be surprised at how little extra input is needed (on a 1:1 at home) to progress between sub-levels in maths.
for example my sons papers showed that he did not answer correctly questions on co-ordinates, measuring the spines of a hedge-hog, and algebra - had he done so he would have gone up a sub-level (subject to teacher asessment).
Now he's not super-bright, and i'm in no way a primary school teacher, but I do think if I had gone through these areas several times with him over this year he would have understood them.
So I do think children are capable of quite a bit more.