The terms 'International' and 'Roaming' are often confused.......be careful! 

For the avoidance of doubt, 'International' means activity originating in the UK¹ but directed to a destination outside the UK.

'Roaming' means activity undertaken anywhere outside the UK. 

To prevent the ability to make International calls², from the dashboard of your my.plan account, go to:

CONTROLS > DATA, CALLS & TEXTS > INCOMING & OUTGOING CALLS > INTERNATIONAL CALLS > [select user] > BLOCKED

INTERNATIONAL CALLS (EXCEPT ROAMING)

International calls (except roaming) restrictions prevent a service from making international calls. This includes calls from the UK to non-UK destinations. The only exceptions to this is while roaming, the service can make calls in-country e.g. America to America, or calls to the UK while roaming e.g. America to UK.

To prevent the ability to make International calls except while roaming², from the dashboard of your my.plan account, go to:

CONTROLS > DATA, CALLS & TEXTS > INCOMING & OUTGOING CALLS > INTERNATIONAL CALLS  (EXPECT ROAMING) > [select user] > BLOCKED

¹   for billing purposes, out of zone calls made when roaming are classed as International calls. Calling the USA from France, for example. 

 ² available to Gold & Platinum account users and Essential account users via an upgrade.