As I am gonna travel abroad I would like to know how to keep using my internet services (email, BBM and web brouser) over a wi-fi connection having the data services turned off to avoid high fees by the phone carrier.
set data roaming to off should be enough its in the same page as mobile network settings, last time did it thought i Had to switch off mobile network to get wifi to work (as you do not want to have "While roaming" set to yes as you get charged soon as you lose wifi)
It is also always a good idea to get a small data add-on for roaming in case you need it or accidentally use some data. It will change your data rate significantly, depending on the country.
If you have Universal Search, type in Manag.... and Manage Connections should show up. Otherwise locate it. Topmost option of "Mobile Network" and you can turn if on or off. I used that a lot in Italy in Jan/Feb. But as suggested, I did add a global data plan for when I wanted to check email or browse and I did not have wifi.