Mobile sim card

I'm travelling around Europe for a month in a few short weeks, can anyone help with advice on what mobile sim card I should purchase for my iPhone 5S while there. I mostly need it for internet connection and will be travelling through London, Paris, Spain, Croatia and Greece. Not sure how your phones or servers work there but something that provides me with at least 2 gb of data over a month??_or if you could explain what your prepaid contracts include.

Thanks in advance. SH