How much data would you recommend getting for a Samsung Galaxy S2?

So no new smartphones with shockingly awesome features worth splashing out £500+ have caught my eye this year, and as a poor, struggling student, my father generously offered me his old Samsung Galaxy S2 for free. I'm going to get a sim-only contract for it, but I'm not sure how much data to get for it.

I'm going away to a different city (and travel a lot around the UK anyway), so I'll probably be using the Google Maps app a lot, will most likely check my emails a handful of times a day, perhaps a few basic Google searches, and check train timetables. That sort of thing. I'll have WIFI in the dorms and most places on campus a five minute walk away, so it's just a matter of how much internet I'll need when I'm out and about.

Thanks all :)