I have limited experience with BA, and I'm just wondering if it's possible to upgrade using miles either at check-in or in the lounge or even onboard?

I'm looking to upgrade 2 seats on a flight from LAS-LHR from CW to F. There's only 1 award seat available on the date I need, but I'm just wondering if there are any available revenue seats left at check-in, will they let me upgrade then? I've always been able to do this on EK, but I don't know if BA work the same way?