The academic experience actually diverge widely for students studying abroad, even within programs. There are many abroad programs at Oxford and Cambridge that are not integrated into the university and are taught by people who do not hold official posts in the university. For example, I was invited to apply for a teaching job, offering tutorials to US students and I have never taught before. A friend of mine is being paid handsomely to do just that this summer and he has never taught at Oxford before that either (Certain graduates do give tutorials after training, though as first years this past year we both havent being trained).
When I studied abroad, I considered the program I did to be excellent: Butler University's ISA program. You are integrated into the colleges and you are looked after directly by fellows of your college. My professor at UCSD at the time had friends at Oxford who were willing to tutor me through personal connections so I was able to expand beyond what my college and the program had to offer. Some programs house you away from college and are not able to provide the best tutors so please do your research.
Lastly for academic rigor, it is actually often easier for abroad students at Oxford especially compared to home students because british students study according to a strict curriculum and what they will be tested on. Since as a visiting student, you dont really sit for official exams, you are more free to pursue your interests but as result, you do not experience the same level of academic pressure. That being said, the tutorial system at Oxford is very different from UCs. It is harder because you have to absorb much more material in a shorter amount of time. As an Poli Sci major, you will be given a reading list often 10-15 books you have to race through it in a week (if you have two tutorials that week, which is possible, your workload doubles) and compose an essay and be ready to explain/defend your argument in tutorial. It could be daunting at first but it gets easier as you progress.
Thats my two cents and I hope its helpful.