I can't really speak in terms of the subject itself because I know very little about political science or Law, but as for the location and uni experience:
I would first of all recommend visiting Reims and London if you can. You have to know that London is going to be very different from the tourist experience; reality is that living in London is expensive, it can be dirty, busy and sometimes lonely. But some people love that and really thrive in London. If you are very independent and don't have a problem with coping by yourself, having friends who live at different ends of the city, sometimes far away, then I would recommend. Reims on the other hand is likely to be a very closed environment. You will probably know most of the people there after a year or two, the clubs and bars will have the same students etc. Once again, you may like that (I do), but a lot of people don't, which is also fine!
Whatever you do, they are both incredible schools so there's no 'wrong' choice. But if you think about what type of environment you prefer, it might help