Why doesn't the lower end of GCSEs just measure the functional skills?
Those maths skills on the functional paper are to some extent part of the GCSE. However there were plans a few years ago (under Labour) to have a separate functional skills paper and students weren't going to be awarded a C in GCSE maths even if they passed the maths paper if they didn't pass the functional skills paper. There was a pilot, and it was all set to launch, then the whole plan was dropped. Suspicion was that too many pupils failed the functional skills pilot and there was too much disagreement about what the papers should look like.
There has been report after report into maths education (including the one for Gove led by Carol Vorderman) which recommended two maths GCSEs, one numeric and one more algebraic, with the majority taking both and the weakest only taking the numeric one. Gove said no, he wanted an academic education for all. The only nod to this was making maths GCSE double-weighted in Progress 8.