Update
As I was saying in my original post, both LH and TK provided the confirmation numbers for the flights they operate.
So I called the UK number for United, and here is what they told me. The rule (apparently with TK) is that United cannot issue the ticket before 24 hr despite TK provided its confirmation code. United cannot issue the ticket because TK has 24 hrs to come back and cancel its flight segment should they want to do so.
The agent told me to call back 24 hrs after the reservation is made and they will go ahead and ticket it on the phone.