Gurney, according to Wikipedia. Britannia was designed so that it could be converted into a hospital ship in time of war but it was never used in this way.

I'm not sure how it works but other ships also can be called up to serve in time of war. SS Canberra was one of a few that did serve during the Falklands war. for example.

My guess is that HM's own would always have been very low on a call up list for wartime service.