Free lessons and practice for school-age and adult learners on Khan Academy

Khan Academy is a non-profit online learning platform offering completely free video lessons, practice exercises and progress tracking for maths, science, computing and more — suitable for primary/secondary school students and adults revising exams or learning new topics. UK residents can sign up for a free account to save progress and use teacher/parent features; content is free globally and does not require payment, though donations support the service.