Hi there
I understand the urge to change courses but I would absolutely advise against doing so solely because of what other people think. It's your life, your degree and your interest- nobody else is going to come and help you in your studies and if Theo/Phil is what interests you the most, then go for it. You don't need any specific degree to go down either the law route or the diplomat route. Law firms recruit almost half the trainees from non-law backgrounds and you can get theoretical legal knowledge from the GDL and SQE anyway. The same goes for the diplomat route, many people do tend to study IR or politics because it gives you more of an understanding of foreign relations but honestly if that's a route you're truly passionate about, I don't see how studying another subject can possibly deter you from still exploring that field. Also you don't need any specific degree for a Masters in IR at Durham at least, other unis may have different requirements. A degree in the field of Social Sciences is preferable, however, in assessing the application, other facilitating factors such as relevant experience are taken into account in lieu of this requirement.
But in any case, if you decide to change courses, you can do so after starting your degree. You'd have to file a request with your current and prospective departments and they can advise you on the procedure. I'd suggest doing this during freshers' week or in the first week of Michaelmas term so it can be done before it's too late. It's not guaranteed that your request will be accepted but it's worth a try!
-Himieka