The Chinook pictured flying over Wideawake at Ascension Island is BP, which had been off-loaded.
BN was indeed the only one of the four remaining Chinooks on Atlantic Conveyor to survive the Excocet atttack. Incidentally, the aircraft that nearly got away from the aft deck (most were stored and flown off the central deck area) was BT. Although it is often written that only 3 whole Chinooks were lost on Conveyor, this severely underestimates the very large stock of spares, components and engines that also went down with Her. Indeed if you were at Odiham or Fleetlands at the time, you would find quite a few stripped airframes as a testament to this. The loss of the Conveyor therefore had a much more fundamental effect on the RAF Chinook force which lasted for many years after the end of that far away conflict.
Having reformed as the RAF's first Chinook sqn in Sept 1981 at Odiham, 18 Sqn did indeed remain based at Odiham until mid 1983 when it moved (at last) to its long intended home base of RAF Gutersloh, before relocatimg post Cold War to Laarbruch in 1993 once Gutersloh was handed over to the Army.