If you intend to come to the UK for Sixth Form, most state sixth forms are free - however you will need enough money to live in the UK for 2 years as a teenager on your own and to be able to have a convincing enough case to get a student visa. State schools dont offer scholarships to overseas students.
If you intend to go to a UK private Boarding School then you would need either a great deal of money - tuition fees alone cost £20K+ a year - or a full scholarship for an international student that covers all costs - fees and boarding costs - and these are very rare.