You can enable this via your online account. Please click here to log in and follow these steps:-
Online account > Plan and device > Manage services > Going abroad
If it shows it's turned off then please turn it on. You'll need to wait 2-24 hours for this to be successful.
You'd also need to turn off International call barring waiting the same 2-24 hour period. This will then enable the service to be used abroad.
If you're abroad and roaming isn't working at all for you, the same team will be able to look into it for you. They can be reached on +447953967967. The team can turn this on from their side. Once this has been done, give your phone a hard reboot (popping the battery out of the phone while it's turned on ) to allow the sim a connection refresh.