How do the standards of GCSE and A-level differ?

At GCSE level, competent engagement with the historical background and a decent argument are sufficient to secure a good grade. 
At A-level, the phrasing and structure of the essay becomes more critical, requiring each paragraph to work towards building the overall outlook of the piece. It will also require the meshing of the source material (if a source based essay), and not simple linear engagement. Use of some historiographical opinion can also elevate the essay to the higher grades.