how important is your subject combination in applying for UK uni courses? Am I too late?

I’m taking subjects that are mostly sciences for A levels from SG (Bio, Chem, Math, Econs) because when I was deciding on my combination, I wanted to keep my options open for uni. But now I’m zeroing in on political science and it would be great to have a chance to study overseas in competitive unis like UCL, LSE or warwick. I don’t have extracurriculars relating to such a field either. Am I too late to start trying?