Hey, A levels are different from GCSEs as you are well aware so revision is definitely needed.
Thanks for reaching out, what a fascinating combination of subjects, especially the EPQ!!!
What's angle you are looking at Thatcher from?
For History, I have created a mind map for each chapter, I do wars of the roses, international relations 1890-1941 and civil rights in the USA.
These mind maps include all key info, dates, people, events and are usually on 1 A4 piece per chapter.
Then I do active recall where I basically do look, cover, speak to myself and then blurt all the information out onto a blank page from memory.
A friend uses Gizmo as do I which is useful for flashcard revision, it is an easily downloaded app. Quizlet is also a popular one.
And of course practicing essays is always a sure fire way to improve and ask your teachers to mark them.
Hope this helps

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