It's Complicated(tm) - Scotland and England are, currently, both part of Britain. Historically, not so much. And they were often far-from-friendly, with wars fought and everything. In a referendum 10 years ago, ~45% of Scottish voters wanted to separate again.
Scotland and England has been the same country for far longer than say the USA has even existed.
My own reaction to the most recent Scottish independence referendum was pretty much "Oh well, at least we're still in the EU"...
They are both part of the United Kingdom (which is the union of the kingdoms of England and Scotland).
Great Britain the name of the island both are located upon. England and Scotland will always be a part of Britain, because that's a geographical area, not a political one.