Most 16–18 schooling in the UK involves some haram, like free mixing. The law says under-18s must be in education, but it isn’t strictly enforced. Is it permissible to leave college and focus on work or business?

This is your decision, and you can choose what is better for you, especially since you know the laws of your country more. However, if you are asking if it is a valid thing to do, and if having a thriving business that you want to put more focus into is a valid reason, then yes, that is a valid course of action. Ultimately, it is your decision as you know your situation better than me.