Zend certified PHP/Magento developer

In Magento1.9 is there a way to override existing overseas customers from selecting shipping address same as billing at one page checkout?

Our client has a multisite which contains 5 individual sites selling different products. The client is based in Scotland and for years has been shipping worldwide. However, after Brexit, our client has decided to stop shipping overseas entirely and now only delivers within the UK and Islands.

As you can imagine various changes have had to be made to the websites ie logo images, removal of text etc. but these aren’t so difficult to amend. The challenges have been at the onepage checkout for billing & shipping addresses. I have already managed to stop county/state from being a required field and also removed it from the form (I still have one minor tweak but it’s not seen as urgent). Another task was to enforce the Country field as showing all countries for billing and only the United Kingdom is available for shipping.

Now here is the dilemma.

An existing registered customer who lives overseas was still able to place an order when selecting the checkbox ‘use billing address’. In the attached image the message clearly states that items are no longer shipped internationally from the UK. The customer chanced his luck but as this is now a very busy time of the year, our client would like to resolve this, if at all possible.

When I ran a test order on one of the sites I selected checkout method as register or guest, filled in all the billing address information as a test customer in France and when I pressed the continue button the country field was emptied, the box was highlighted in red with some text prompt underneath Please select an option (see attached image)

country_field_prompt

This is the kind of validation response I would expect any overseas customer to receive.

My question is

Is there a way to enforce this type of validation for the existing registered overseas customers so that they are unable to process any order for themselves?

This is the file directory path where I believe the code for the validation is made:

app/design/frontend/ultimo/store1/template/mergeinfo/persistent/checkout/onepage/billing.phtml

I am not a coder per se but as I follow the code line by line in the above file it appears to check/validate for a new billing address form but it doesn’t seem to check or validate for existing registered international billing addresses.

I could be completely wrong here but can anyone suggest a solution or point me in the right direction, please?