Hi Lordsmellypants,
Thank you for messaging with us, glad we could help out.
With regards to your questions about buying data packs. With buying data packs you are unable to do so when you are out of the EU, data charges are £5.00 per MB.
You will be able to use WiFi to make calls via applications like Messenger or WhatsApp while abroad for free, but I would suggest looking at disabling the data roaming option to prevent high roaming costs.
We can do that for you before you go alternatively you can disable mobile data on your phone to give more control while abroad.
We do have a safety limit if you chose not to disable roaming data you can find it here.
Zoie