Originally Posted by
Often1
Knowing that you are headed off to Washington without the bag, I would use BA's generous allowance to carry your first 3-4 days of clothing in any event.
Unfortunately BAs policy will not apply here as the bag was lost on a journey with IB that ended in LHR. No bag will be checked in at LHR for BA to be the last carrier on.
OP, hopefully you have a worldtracer (or similar, not sure what IB use). keep it updated with whatever address you need the bags delivered to. The fact you have travelled on is not unusual and its IBs responsibility when they find the bags to get them to you, wherever you are. If you are only in IAD for a few days it may not be worth making that update and just getting the bags delivered to home. Anything you need to buy may need to come from travel insurance from your Spain trip