Unless you turn off the data roaming function on your phone it will connect automatically. Instuctions for doing this on an iPhone can be found on the following page. This also shows you how to turn off mobile data for individual applications.
The proliferation of free wifi hotspots in hotels etc. does mean you will still be able to use your phone for email but you will not incur additional costs.
It is possible to add an option to your tariff that allows you to use your UK allowances while abroad, details of the charges and countries are listed below.
This option will charge you every day you use data when abroad.