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. A green message will appear. You'll need to wait 2-24 hours for this to be successful. Once activated you may be required to restart the handset to get it connected.
I hope you enjoy the rest of your holiday. Please let me know how it goes.