Hi! I'm also an aspiring law student and I currently take A Level English Lit, History and French. I also didn't do History at GCSE, so if you are interested in it, I recommend speaking to the history department at your sixth form / college to see if they can take you onto the course. If you do decide to go for it, ask about what A-Level spec they're on, have a look at the topics, and find out if prospective A-Level History students who have done it at GCSE have already covered the basics of any of the topics. For example, everyone in my class did Elizabethan England and the Cold War at GCSE; I went away and learned the GCSE content for those over the summer so that I would be well-prepared for my modules in Tudor history and the Cold War.
You've got a solid selection of A-Levels though, and I think it would be wise to just not take Maths. As the people above have mentioned, you don't need to take 4 A-Levels; universities will only look at your 3 highest grades, so there is no point putting in energy into a 4th A-Level that you may not even enjoy and that might not count towards much.
Best of luck!