you need to remember as well that alot of the stock apps use internet! so be careful, because you'll either use all your credit up looking at what the weathers like in botswana or you'll tan it looking on google maps! the contracts designed by o2 give u free. well not free but it all come in with the monthly plan!
"scudder" is totally correct in saying that u should buy a different sim card to each country you go becuase it will keep the charges down, other than that you can purchase a sim card from the uk from a place called "
http://www.phones4u.co.uk/?ppc=true" i dont know if you can buy it from this website but you can defo buy it in store!! its called """"" Lebara"""""" you can even see it at
http://www.lebara-mobile.co.uk
hopefully this will help, let me know how it goes!!