Secondly, Political Science does overlap with Constitutional Law. But, the dimension of C.Law is far more expanded. If you are lucky, you might be asked the same questions as Pol Science. Otherwise, the examiner of C.Law has a lot of topics at his disposal which he uses to make the paper very different. See 2021 paper of C.Law.
International Law goes well with IR. However, its score for the past few years has remained in the range of 40s.
I would recommend you to go for C.Law but devote considerable time to the subject ( a month or more). This will strengthen your Pol Sci as well.