For what I know BTECs are equivalent of A levels, so presumably they were designed with a view that everyone should get GCSEs. Maybe the C&G exams were forgotten because they were judged inadequate for the modern world. I don't know.
You need to decide what level of education is appropriate for a citizen that has the right to vote, to elect politicians which will define the future of their children, decide on euthanasia, cyber-surveillance, unmanned killing drones, climate change. They are already buying Google class. These kids will need to understand what is going on. Otherwise you kill democracy by ignorance.
You can't have a system with 60% success rate. It is not a good benchmark in any industry.
You can't have a system where 40% of 14-16 years old spend 2 year frustrated at school and get out with nothing to become NEETs. These two years put them off learning and hard work and undermine their employ-ability. At 16 people should have some sense of self worth, some qualifications, something they are good at that they can use directly on the job market, in further education, something respected and accepted.
If 20% they can't pass GCSEs, they should be doing BTECs at 14, not after GCSEs. After all the taxpayers are paying for 2 years of study and that should not go to waste.
Then GCSE should be the universal exams at 14? Then you separate into vocational and academic streams. 6th form would then be 4 years.