Data roaming is simply a phone setting which allows you to use mobile data whilst roaming.
Of more importance is that your SIM is enabled for roaming - which enables access to foreign networks, without which your phone will not get coverage of any network abroad. This will prevent data roaming, voice roaming & text roaming.
If you're on PAYG, your phone is auto-enabled for roaming once used in the UK first.
If you're on pay-monthly, this is normally done by texting ROAMING to 150 before leaving the UK.
