The Key Stage 2 tests no longer include a grading for Level 6. Up until about 4 years ago, high achieving pupils could sit a separate level 6 paper, which in practice was very rarely achieved, therefore it was abandoned. It is a shame for those few children who are capable of achieving it, but I suspect it was seen as a 'waste' of money.
My son is now in the Upper Sixth and he did sit the Level 6 maths paper in Year 6. It was extremely hard and he missed the Level 6 by 3 marks - the school were really pleased but he was disappointed. I wouldn't worry too much about achieving level 6 at primary school - they get the chance to prove themselves at secondary school. My son is doing maths A level and got an almost perfect score in his AS exam (294/300) so he has proved he has a talent for maths. Shame he doesn't want to do maths at degree level!!