As Head of the Commonwealth I believe she has "royal title" over all the land within it, including Canada and Australia (and the Falklands), which is what makes her 'nominally' the world's largest landowner. In practice, of course, that means F-all.
Whilst that is 'nominally' true, how far would she actually get if she rang up Fox & Sons one day and said: "One would like flog Australia. See what you can get for it"?