I took a look at the ship technical drawings. The hull is incredibly thick!
Yep! They were "Bomb Ships" originally, build to support heavy coastal bombardment batteries during the wars. After Nappy was shipped off the the island (the second time), many of these especially-tough ships were repurposed by the Royal Navy for polar exploration.
That whole story of the RN looking to use its war surplus in interesting ways while also giving the surfeit of listed officers something to do is a saga in and of itself. Look up John Barrow, the (then) post-war Secretary of the Admiralty if you're curious.