The Feynman Lectures on Physics are a three volume book collection covering almost all pre-1930's physics.
They make for fantastic bedtime reading for any Physics Undergraduate, and are a nice example of 'supercurricular' reading for Year 12/13 students looking for something to write about in a UCAS personal statement! (The intuitive introduction to vector calculus in Volume II is particularly accessible for sixth form students!)
This site contains an organised collection of some of Professor Leonard Susskinds physics lectures given at Stanford University.
As Professor Susskind describes himself: "The courses are specifically aimed at people who know, or once knew, a bit of algebra and calculus, but are more or less beginners."
Therefore this resource is well suited to Year 13 students as well as early Physics Undergraduates.
Nobel Prize winning physicist Gerard 't Hooft presents an excellent roadmap of topics needed to start doing real theoretical physics. Under each topic, Professor t' Hooft provides a breakdown of what he thinks are the important areas to focus on, and numerous helpful resources (lecture notes & textbook recommendations).
The people to whom the material on this side is useful quite easily ranges from keen GCSE students, all the way through to advanced undergraduate and graduate students. That being said... Many of the resources chosen by Professor 't Hooft are not the easiest first look at a subject, and may scare off some students that haven't already seen some of the material presented in a less formal way.
For that reason, I'd recommend this site to beginning undergraduate students (and up).
I used to watch this Youtube channel a lot when I was in Year 11 through to Year 13. He has some great A level and GCSE revision videos, but also a lot of honest introductions to subjects like Quantum Mechanics and Nuclear Physics. A lot of the content in the more advanced videos takes some digesting, but (perhaps in part due to nostaligia) I really enjoyed these videos as a first exposure to some advanced topics!
Another channel I watched a lot of before applying to University! (and still watch today)
Videos on a wide range of Physics/Astronomy topics from researchers at the University of Nottingham.
You should also take a look at Brady Haran's other channels: 'Periodic Videos', 'Numberphile', 'DeepSkyVideos' and 'Computerphile'.