What is the most effective way to prioritise what you need to learn or revise for in the exam?

The chief mistake made here is in believing that a History A-Level exam is about how much you know. If this were true, exams would be a blank time-line for you to fill in. Instead, you must remember that an exam at A-Level is far more focused on what you know in very specific areas and chiefly whether you communicate that to an examiner in the correct way.
Keeping this in mind, the most effective revision strategy is to use past papers and examiners reports to identity the specific areas you are expected to know about, and usually these areas are surprisingly small, often little more than some dates and a basic fact file about events on said dates, and focus heavily on memorising those inside out. Use tables, flash cards and other seemingly simple methods to cut away the superfluous information, and then, what you have left will be the key priority of your learning and revision. At A-Level, the difference between success and failure is not how much you know, but the technique of how you write and so this, not information revision en-mass, should be the priority.