"but wonder if GG is seriously suggesting we build all over our countryside?"
That is a slightly weird leap of logic.
Jalima you are right. I should not scan articles
It actually quotes the figures you do for England, Scotland, Northern Ireland and Wales. It also states a total for the UK:
"Five hundred experts analysed vast quantities of data and produced what they claim is the first coherent body of evidence about the state of Britain's natural environment. Having looked at all the information, they calculated that "6.8% of the UK's land area is now classified as urban" (a definition that includes rural development and roads, by the way)."
But it turns out urban isn't urban as we think of it. The article says that "just over half the land (54%) in our towns and cities is greenspace - parks, allotments, sports pitches and so on." and adds that gardens are another 18% and rivers, canals, lakes and reservoirs 6.6% of that designated "urban"
When they compute all of this it turns out that
"In England, "78.6% of urban areas is designated as natural rather than built". Since urban only covers a tenth of the country, this means that the proportion of England's landscape which is built on is … … 2.27%."
In the rest of the UK it is even less.
So while I apologise for letting my attention wander and misquoting Mair, I think you are pushing it to say we will not be able to build sufficient houses.