Are you sure? I see availability as PAS-ATH-LHR
with OA/BA.
Anyways let's not reminisce on what could've been done.

hco has a good point, what's your ATH-LHR ticket value? What would be the cost of sending a 23kg luggage to London?

Next time, issue your tickets through a third party, OA might not be able to go and play in BA's inventory, but a TA sure is