It really depends on your interests, universities usually let international relations students choose modules from the school of law. Both degrees are very employable so you should choose what you enjoy. International relations isn’t confined to politics as you can work in international economics and media. Law can usually get you into the same careers as an international relations degree e.g. the civil service/NGOs/IGOs. If you decide you want to be a lawyer after studying international relations you’d probably have to do a PGDL. Depending on where you want to work, having a master's degree may be useful as it can open a lot of doors for you when it comes to international relations