I've booked a TP run IAD-LHR-MAD and same routing on the return. I arrive in MAD mid-day and am booked to fly out early the next morning back through LHR to IAD.

What is the likelihood instead of spending the night in MAD I can arrive in MAD, go to the BA desk and return to LHR to spend the night in London and fly out on the same LHR-IAD flight as originally ticketed? Assuming there are available seats, would that incur a re-calculation of the fare or (fingers crossed) only a change/re-booking fee? I'm BA Silver if that matters.