How to narrow down focus in humanities/social sciences? I’m looking to enroll in a social studies/humanities undergraduate course, but I’m unsure of which subjects to focus on in my studies. I have ambitions to progress on to postgraduate study, research and academia; industry and financial gain aren’t on my radar.

Philosophy, politics, history and international relations all seem good fits, that could open up lots of opportunities, but how would you decide which subjects are most aligned with your interests, and crucially which combinations would be feasible and most facilitate further study/research?
Ultimately it's up to you to decide what you are most interested in. No one can answer that for you. That said, it's very important that you get an idea of what research is like in all of these fields, as it is often very different from what popular conception or even undergraduate coursework is like.

Assuming that you are relatively early in your undergraduate studies:


Take a variety of classes in the fields you are most interested in. Go farther than just what the class requires: go to office hours, talk to your professors, think about and discuss other questions of extensions related to the coursework, dig into any references that are mentioned in lecture, etc.
Find a faculty mentor(s) to talk about these issues with. This might first take some narrowing down of your interests, but a faculty member should be able to give you some guidance on how your interests fit into academia and an idea of what academic work in that field is actually like. This will often be a professor from a class, but many departments also have a designated faculty member that handles the undergraduate program who may be available to talk to. In my experience, most academics will be excited to talk to you if you express interest in further graduate study.
Try to get some actual research experience. This is heavily field-dependent and I am not very familiar with research opportunities in the humanities, but hopefully you will have some faculty members that you can talk to about this. If you are in a field that doesn't often use research assistants, see if you can do a faculty-directed independent study. 
Start familiarizing yourself with some of the academic customs in the fields you're most interested in and start reading some actual research. A good place to start is often the webpages of the faculty at your school that you are most interested in. Read up on their latest publications/books/etc. and use that as a starting point to get at least a small idea of what kind of work is being done and what research looks like.
Find some current graduate students at your school and talk to them about what kind of research they are working on, how they go about doing it, and why they chose that field.