More reading about the 1953 coronation provides this look into the security for the Regalia overnight at the Abbey.
The evening before the special day a small van and one police car drove into the Dean’s Yard at the Abbey.
The two Crowns and the Regalia had arrived from the Goldsmiths’ and Silversmiths’ Company.

Eight Yeomen Warders from the Tower were the escort to the Jerusalem Chamber where the crowns, swords and sceptres were placed on a table.
The Yeomen Warders, traditional custodians of the Crown Jewels, strapped on pistol holsters over their Tudor uniforms.
Fifteen Special Branch men also spent the night in the Abbey. Security was tightened outside around the Abbey.

My mind boggles attempting to begin to think of just what the security arrangements might be in this day and age.