There were four relatively modern heavy ships sunk, the German BC was lost to uncontrollable flooding forward.  The three British BC were lost because they were packed from the turret to the magazines with uncased cordite charges with igniter pads leaking gunpowder.  Cordite, if it began to deteriorate (and it could do so very quickly) had a tendency to explode if exposed to shock, and a shell through the turret armor would more than provide that.  So, lack of deck armor didn't have a thing to do with the loss of the British BC's.  the lack of turret face/roof armor did have a great deal to do with it, as did unsafe cordite handling.  Now had the BC's been subjected to long range plugning fire it would have been another story, however the armoring system until USS Nevada didn't provide for in any BC or Dreadnought design.