Did the Royal Navy consider radar-picket submarines?
Short answerYes, but it rejected the concept quickly because the curvature of the Earth severely limited a submarine's low-mounted radar horizon.
NotesA submarine can provide stealth and forward positioning, but its low silhouette also leaves the radar close to the sea surface. That geometry sharply restricts warning range against low targets. The concept therefore loses much of the reach that makes a radar picket valuable.