Hi Gemma This isn't a feature of the cart but can you try this in your includes.php file and see if it does the trick...
if(@$_POST['mode']=='go'&&@$_POST['country']!='1'&&(@$_SESSION['clientID']==''||(@$_SESSION['clientActions']&8)!=8)){ print '<div style="margin:50px;text-align:center">International only for trade customers</div>'; $_POST['mode']=''; }
Of course, changing the message to something by the marketing department. And please test a lot and if it doesn't work, take it out and let me know.