I didn't take A-Level Politics (didn't have the option unfortunately) but I had the same issue as you when I was doing my A-Levels. In Economics we were so short of time anyway, and I often would fail to finish writing essays, even though I knew what I wanted to say. My teacher's advice at the time was to change the structure of my writing. For our essays we would have three paragraphs (+ intro and conclusion), but that would take too long. So instead they suggested some of us write two longer central paragraphs instead of three. So we would have to develop our ideas a bit more, but it would save us time. Perhaps you can ask your teacher if they have any similar suggestions for Politics essays?