Take a look at what constituency the uni is in, who holds it, and how long they've held it. That should give you a good idea for what sort of people you'll find there. Then remember that wherever you go, you're with students, and students love to get protesty about everything, so political activism is almost guaranteed. Even if the demonstrations aren't in your area, you'll probably find people willing to drive a coachload of you all over the country.
Although, yeah I'm with neal95 on Sheffield being the most politically active.