Black Buck One's planned ingress did rely on an element of surprise to enhance its chances of survival. Intel suggested that the airfield was defended by a combination of 35mm Oerlikan guns and/or Roland surface-to-air missiles. Black Buck 1 also benefitied from carrying the Westinghouse -10 ECM pod on one of the Skybolt pylons that was primed with a deception jamming programme. Remaining above a sanctuary height of 12 000 ft assured it of a low risk of attrition to surface-to-air weapon systems.
Black Bucks 5 and 6 were an entirely different matter though. Both Skybolt pylons were taken up by Shrike AGM-45 anti-radiation missiles and so Vulcan 597 was not afforded the protection of any ECM capability. An element of surprise was only required to ensure that Argentinian air-to-air defences were not pre-alerted and that Black Bucks 5 and 6 were not met by Daggers on CAP.
Following ingress at low level and a climb to 12 000 ft, Black Bucks 5 and 6 had to announce their presence and go active to try and entice Argentinian defences to illuminate them. An ARM such as Shrike needed to home in on an active radar to be effective. Indeed, Black Buck 6 descended from its sanctuary height into the MEZ from a 20 degree dive and bottomed out as low as 6 000 ft so as to entice the Sky Guard radar to illuminate Vulcan 597 and the Super Oerlikan guns to open fire. In this manner it was able to gain a firing solution on the Sky Guard fire control unit and destroy it with a direct hit.
Any intel that the Soviet AGI provided would have enabled the Argentinians to man a CAP at the appropriate time if they decided that was a priority. Or was it more important to retain the Daggers for defence of the mainland?
The full story of these missions can be found in the link I provided earlier to the chapter in 'Vulcan Boys'. The first time in over 30 years that the truth of these missions has finally been recounted.