According to the linked paper, the UK would require 3 million hectares to provide sufficient calories (assuming everyone ate nothing but barley stew). It also points out that, at least in 1975, that's more or less what happened - the UK grew enough barely in that year, using 3.6m hectares, to "feed" its population (at least, in terms of calories needed).<p>So - it's already true! We grow all the barley we need to sustain ourselves, and the rest is given over to other produce that provides the other nutrients we need, as well as some luxury. Yay vegan sustainability!