Pro Forma / Pay On Account ONLY for International Customers

[SteveWinch5DS]

Hi Shopify Community
Long time no question! 
For our Trade accounts, we want to stop them being able to pay for the orders up front and only be able to Pay On Account, due to us having to work out shipping costs prior to payment being taken.
Currently all out Trade customers have both options in the Checkout.
Anyone know how we can remove all payment options except Pay On Account for our International customers please?! I've been having a look and can't see if it's possible or where I need to look!
Many thanks

Steve