To my knowledge this is the island that once held Sealions and even earlier than that, Chimps but during the daytime only. I am not 100% clear on this though.
Yes various pinnipeds were kept here: in additition to the California sea lions, this enclosure has also housed southern sea lion, common (harbour) seal and grey seal.
@Pertinax
Since my last post, I have checked some old Whipsnade guidebooks; the 1939 and 1940 guides both state that: "In the centre of the island is an electrically-heated house" so presumably the chimps were housed in that building overnight.
@Tim May I also looked for the brick 'kennel' type housing nearer the road boundary that held smaller cats and the Spectacled Bears at one time, but could not find it. Maybe that has been demolished now?