Am I correct in interpreting this photo as showing one thrust reverser engaged, while the other is open? Do the thrust reversers fall into place on a fully powered down Concorde? Could this be caused by an engine being removed for display? If so which engine was removed?
Yes the left (#1) reverser is engaged in this photo. G-AXDN is the British pre-production Concorde and has some differences from the final versions. G-AXDN contained internal clam shell reversers, if the GIF at the bottom of this informational page about G-AXDN is any indicator that external engine reverser may still actuate for show and in this case was simply closed while you were there. This video shows the light up ring that they may have used in the GIF (5:02)