Another film about what we would to survive is 'The Last Valley'. Michael Caine rightly regarded it as one of his best performances.
'The Wild Bunch' and 'Lifeboat' are films with a similar theme. Characters are faced with the choice; should they sacrife others, to save themselves.
By the last stage of 'No Blade of Grass', that is all that is happening. A couple where one is too sick to walk, a couple who have just given birth, a couple with a baby still breastfeeding, Finally his own brother.
Who isn't John Constance willing to sacrifice for the sake of the group?
The Sock