Compulsory education (possibly alongside work) until the age of 18 was passed into law under the Education Act 1944 but the relevant section was never brought into force. The Government finally caught up with those plans over 60 years later, with the passing of the Education and Skills Act 2008. That legislation initially extended the length of compulsory education (whether at a school or college or as part of a traineeship or apprenticeship) to the age of 17 and then to 18.
So everyone up until the age of 18 is meant to be in some form of education. However, just as every child between the ages of 5 and 16 is meant to attend school (or to be educated at home) but not all do so, there will be some young people who don't take on an apprenticeship or traineeship, or attend school or college.
However the term 'NEETs' is used by the Office for National Statistics to refer to people across the 16 to 24 age range, rather than just to 16- to 18-year-olds, so there will inevitably be a somewhat larger number of people in that group than just those who aren't in any form of compulsory education or training:
https://www.bbc.co.uk/news/topics/cq0y8j9nlelt/neet