International relations is a part of politics (the second P) at Oxford, so relatively easily I imagine. You can link it more into the economics angle with discussions about international political economy as well. You could also discuss e.g. ethical issues or political theory matters underpinning IR to link to the philosophy side, while still being IR relevant.
Note that at many unis IR and politics are two sides of the same coin, and in some they're purely synonymous as far as degree courses go. Very few unis have a separate IR department (LSE is an example of such an exception).
Note that for most PPE courses you do the same economics modules as single subject economics students, just not as many of them, so you are still going to need to do a fair bit of maths in a PPE course (at Oxford or elsewhere), albeit at Oxford you can drop economics after first year.