Are you intending to leave the airport before boarding your GLA flight - sounds like no, but could be important? If so, you will need to get the ready to fly flag removed from your booking before you will be allowed through security a second time. Go to a desk landside and, in my experience, allow plenty of time for several attempts at this. In that case, your plan A may be easier - arrive in the CCR for your INV flight, but inform BA when leaving the airport that you're not taking the flight.