Why not? She owns it all already ! All the land in her realm is hers. She graciously allows us to hold some of it 'freehold' ,free of the claim of any other citizen to have it or occupy it.
In practice, of course, this is but a piece of legal history, a legal fiction.The only way Her Majesty gets it back from you is by Her Majesty's government doing so by compulsory purchase, with some compensation, under a bill made law when Her Majesty assented to it becoming law. Parliament and Her Majesty's government make the decisions and it's Parliament that has the real power in the land. Unless she has ambitions to suffer the same fate as Charles I (executed ) or James II (driven into exile) and has forgotten how William III came to the throne ( replacing James II, effectively on the invitation of Parliament, and on Parliament's terms), she'll neither interfere nor try to! In wartime Parliament may decide have laws to seize land, for the defence of the realm, but she won't be ordering it or doing it.She'll just give her assent to whatever bill is necessary to that end.
So the film was making a joke, one with a grain of historic truth, but a joke nonetheless.