More on level 5/6 at the end of KS3:
In literacy, the climb in difficulty between KS2 scores (level 4+) and KS3 scores (level 5+) is very steep. This is because to attain 6+ in reading, for example, pupils will have acquired the ability to analyse a complex multitude of written forms, including canonical literature. They will also be able to write about those forms and their historical context in fluid ways, using analytical techniques that do not come easily at all.
I work at a grammar, and believe me, none of the pupils come to us ready to analyse literature - especially poetry, writing about which they almost universally loathe. Getting them to use the point-evidence-analysis technique (PEA, or PEE, or PEEL, or whatever any given school's version is) is a long and draining process. Without successfully completing that process, however, they cannot climb above a L5.
IMO, literacy and writing-about-literature should be taught as discrete subjects. Lit should be booted out of English language GCSE too. Fight the power! (Or maybe not.)