I think the main advantage of politics A level if you want to do politics at uni is mainly the fact it shows that you're committed to politics, and that's something universities really value. If you have other ways of showing your commitment, then you'll be fine without the A-level.
Plus they rarely, if not never, ask for it specifically cos not all schools do it
It was useful for me because it confirmed my decision that I definitely wanted to take politics, but you also seem pretty sure about that at this moment
So long story short, don't worry too much about it
(If you haven't guessed, I'm going to do Politics at Uni in September) x