Can Exocet missiles be remotely disabled with secret codes?
Argentine Exocets destroyed HMS Sheffield, the Atlantic Conveyor and damaged HMS Glamorgan (Reference). It is evident that fleet protection vessels did not have the ability to remotely disable Argentine Exocet missiles.
HMS Sheffield was destroyed on 4th May 1982
It has been written that Mitterand reported to his psychoanalyst on May 7 that Thatcher asked for "secret codes" to Exocet earlier that day.
The Atlantic conveyor was sunk by Exocets on 25th May whilst carrying 11 helicopters, 14 Harrier aircraft and other vital supplies. The loss of this vessel put the mission into jeopardy.
HMS Glamorgan was hit by Exocet on 12th June At this time, according to the story, the UK had the Exocet codes for more than a month (39 days).
Cost of materiel destroyed
It has been suggested that the strategic value of the supposed Exocet command-destruct codes so far exceeded the costs of vessels destroyed that it was not worth using the codes to prevent destruction of those vessels and crew.
In "Britain & the Falklands War", 1988, Lawrence Freedman wrote:
"The campaign itself, Operation Corporate, is now estimated to have cost about £1.5 billion. The cost of replacing lost equipment is put at £1,278 million. The largest single item in this figure is £641 million for four new Type 22 frigates... to replace Sir Galahad is put at £69 million, and new aircraft account for another £116 million."
Role of destroyers
It has been suggested that only ships other than the ones destroyed were provided with the Exocet command-destruct codes.
A major part of the role of the type 42 guided missile destroyer HMS Sheffield and county-class destroyer HMS Glamorgan was to protect fleet vessels. We can infer that any anti-missile capability would likely have been provided in these destroyers.
Did the British have the command-destruct codes but chose not to use them?
Commenters suggest that the British, having urgently pressured Mitterand into giving up the secret destruct-codes, may have then decided to sacrifice the lives of dozens (potentially hundreds) of servicemen and hundreds of millions of pounds worth of assets to prevent the Argentinians from somehow deducing that the British might have access to secret destruct codes of which the Argentinians were hitherto either unaware or had neglected to change despite knowing how.
The least we can say is that the reported events of the Falklands war provide no evidence to support the claim that Exocets in 1982 had a remote command-destruct feature.
Value of hypothetical destroy-code vs radar info
The report of Mitterand's comments also says that "The most valuable information was on the Exocet’s homing radar." This is inconsistent with the idea that the "secret codes" allowed remote abort.
Fire and Forget
The manufacturer describes Exocet as being "over the horizon" and "fire and forget" meaning that once fired, it does not require further guidance commands from the firing platform, this suggests there is no need for it to be fitted with a remote command system, such a system for command-destruct alone, would add weight and cost for little operational benefit for small tactical missiles of the 1980s designed to be air-launched. Of course it does not rule out the presence of such a system but we can make some assessment of the balance of probabilities.
An Argentine source writes the Exocet had "range: 70 kms (35 miles) Cruise speed: 1100 km/h (Mach 0.9)" - this gives a flight time of 3.9 minutes maximum.
The essence of a fire & forget missile is that the pilot does not wait around to monitor the flight of the missile (with a finger hovering over a hypothetical command-destruct button), the pilot ducks below the horizon and turns away from the target.
Are missiles really built with the capability to be remotely disabled by secret codes?
There are reports that some more recent missiles for surface to air use do have a command-destruct capability.
The capability helps mitigate collateral damage when used in a surface launch role within an urban environment.
(This wasn't the role of air-launched anti-ship Exocets in 1982)