Still confusing. This isn't logical. You said "another cancellation fee". I thought they had indicated they let you apply the FL ticket to the ticket to UK, because your mother had died.
And, at that point, did you not purchase the ticket to the UK?
It doesn't sound logical for you them to let you apply the FL ticket to a ticket to LHR on hold, without completing the purchase
I took "buy out" to mean they wanted you to pay for a ticket on hold, then cancel it and pay the cancel fee plus the ticket price difference, but if AA already had agreed to let you use the FL ticket for a partial LHR ticket payment, I do not understand why you didn't pay for that ticket at the same time. Did you promise to pay for the ticket they had already used the FL ticket as partial payment for? Maybe if you had already started the actual payment process for the original routing to LHR, they might need you to complete that booking, as you had agreed. The change is a different issue, because if you had already had a paid ticket, you do not get a free change. Of course, you can get a refund for a ticket if canceled within 24 hours of booking. I think it would be more clear if you could clarify what actually happened.