Royal Mail shipping address instead of payment address

Hi, I have added shipping address into guest checkout as it's not part of installation in 1.4.7 by changing template > checkout > guest_step_1.php and controller > checkout > guest_step_1.php

The issue now I am facing is that Royal Mail is picking up Payment Address as main reference for calculations. The task is to change it's parameters so that is picking up Shipping Address. At present time if customer wants to pay by Irish card and to be sent to UK, the iso_code_2 country is being picked up as Ireland.

This is something that has to be changed in shipping > royal_mail.php but I cannot find exact line to change.

Any help please?
Thank you in advance.