Considering you want to do Maths within 6 months and you are taking other A-levels: Economics, Media studies and Law, then I guess if you really put your mind to it and grid out the topics, you could have it completed. As maths A-level is a giant chunk, then unfortunately you would have to make some limitation e.g. gaming, social media e.t.c.
However, you could after completing your initial A-Levels think about taking a gap year and focus on your A-level Maths then and after achieving your grade you can apply for the university you love.
After all, its pretty much the same thing but the only difference is that you will be starting your Economics course at university 1 year later.
Hope that helps,
Good luck ! 🙂