Pre-Norman Invasion nobles weren't ransomed.

The 'King's Ransom' concept came specifically from Dickie One's idiot return route from his Crusading.

When fighting occured between the either Saxon, British, Irish, Scots or Pictish kingdoms, kings & nobles were slaughtered.

When the British & Saxon kingdoms engaged in civil war (by far the most common occurance), kings & nobles were slaughtered.

The Great Summer Army of Guthrun destroyed the two fueding Northumbrian kingdoms. He took both kings & re-arranged them until their lungs were on their backs in honour of one of his gods.

Ransom came with the Normans. The Vikings were invading an island to which ransom was a fairly alien concept.