Well the last time I had to cancel a BA ticket I cancelled online and the refund went back into my account within about 2-3 days.
I suppose you could buy a fully flex ticket and offload yourself at the gate, but you'll cause a headache for the staff if you do as they'll have to get you out of the airport; I can't imagine this is going to endear you to anyone very much.
I honestly think if you feel your grandparents are unable to get to the gate on their own then sending them abroad is not a good idea. How are they going to negotiate UK Immigration ? Baggage Reclaim ? Then negotiate US Immigration and Baggage Reclaim on their way back ?
BAH