All of your subjects are traditionally academic subjects, so there is little concern in that regard. In terms of changing route towards science areas, whether you could stay and do a BTEC would be up to your school, however you are funded by your local education authority (or whatever it's called now) until you are 19 (i.e. the academic year ending after you turn 19). So it's certainly possible. However, the BTEC route would limit you greatly in the degrees which you could apply for, particularly at so-called "competitive univeristies".
However, the vast majority of such universities offer foundation year routes into their STEM courses, where you study a preliminary "year 0" and cover the relevant A-level equivalent content needed for the degree. As such, you can just apply to the 4 year (or sometimes 5 year, if they offer it in conjunction with one of their undergraduate masters courses i.e. MChem, MPhys etc) "BSc in X with Foundation Year" without having to take any extra time to do a BTEC etc. I would recommend this route as then you know you are covering all the material necessary to continue into the degree (subject to achieving stipulated progression criteria, which are there for a reason so, not really something to worry about...). Some examples of universities offering such foundations years are Southampton, Manchester, Loughborough, and Durham, among others - quite well regarded universities. Many others do offer such courses as well...
That said...if you are interested in the sciences, and presumably enjoy studying them and do well in them academically, why are you not taking a suitable combination of STEM A-level subjects? You can apply to a degree in IR, Politics, RS, Law, etc with only STEM subjects at A-level (even assuming you take three such subjects). The only degrees which you could apply to with your planned options vs a set of STEM subjects would be History, and joint honours courses with History. Everything else either has no specific subject requirements, or you're not taking the required prerequisite subjects (e.g. English Literature or languages for those degrees). So...it seems like you would solve all your problems (including your concerns about the acceptability of your subjects, although as above that's not really an issue here) by taking relevant STEM subjects of interest.