IR or Philosophy?Watch
As far as graduate prospects go, there isn't going to be much difference between the two degrees since they provide similar transferable skills, with the exception that a politics/IR course will have more quantitative methods/stats background which may be desirable for some roles. You could develop those transferable skills which studying philosophy in a joint honours course as above though (or by taking certain optional modules/papers).
If you are particularly interested in the "practice" of political science/IR as well as philosophy, I would recommend you look into joint honours courses in both subjects, which as noted are widely available (also potentially PPE degrees, if you have A-level Maths and don't mind doing some economics, since usually you can go "bi-partite" on such courses after first or second year and only focus on two of the three subjects if desired). If you are more interested in political theory then you would probably be just as well off doing a single honours philosophy degree realistically.
You may note I've referred to politics and IR more or less interchangeably in the above; that's because in the UK, at undergraduate level, they are in most universities the same course, or at least provided by the same department with only minor differences in which modules are core vs optional. LSE is the only university I'm aware of with separate IR and politics departments which have fundamentally different and separate courses. LSE offers Politics and Philosophy, but not IR and Philosophy. You may be able to take optional modules from the IR department on the PP course though, possibly.