I know that the expected standard for a year 6 child is 4B in key stage 2 SAT. The majority of children easily achieve this level.
www.guardian.co.uk/education/2012/sep/20/primary-school-pupils-sats-targets
80% of children achieve this standard in both Maths and English. (I think the guardian sees 4C as a pass rather than 4B)
What is the mean average score natonally as lots of children do better than 4B.