Some more info from another of Chris Gibson's books. The idea was proposed in 1954 and only the then, under development Lightning and the Javelin would have the performance to deliver the weapon and survive. They needed to be about 4000m away from the 25kt weapon. Delivery would be loft using AI radar.Lack of early warning could mean that the weapon was used over land...this, and other reasons, lead to proposal to use the Bloodhound missile instead.