This really does seem like an impossible situation. You are doing nothing wrong and everything right. You will unable to purchase your ticket online as you are former London Underground staff, you are offering to purchase on board but the staff are unable to print the ticket, you are reasonably not expected to miss a connection at Preston so are purchasing at the first available opportunity.
If Squires Gate has a ticket vending machine, start getting yourself a promise to pay. You shouldn't have to do this, but it just an extra insurance in case you come up against difficult staff, as you will have a printed authority to travel.