The best way to do this is to amend field that shows this shipping method on the invoice to show different text if the country is not the UK.
To do this :
1. Display the Printing Fields dialog (select Tools->Options menu option and select the Printing Fields tab).
2. Click on the Add button.
3. In the macro to use type in [%POSTAL_SERVICE%]
4. In Value type in NON UK [%POSTAL_SERVICE%]
5. In Only use this macro if field select Delivery Country
6. In the value field enter # United Kingdom
7. Click on OK.
Now when the postal service values is displayed on an invoice if the country is not United Kingdom NON UK will be added before the shipping method. You can change this text to whatever you want.