Really? Check this: https://www.bre.co.uk/filelibrary/pdf/rpts/countryfactfile20...

Page 3, Summary:

Around 83% of homes in each country have some double glazing. Northern Ireland has a higher percentage of homes which are fully double glazed, 62%, compared with 32-44% in the other countries.

So, ~60% of the houses are NOT fully double-glazed. In 2007. Srsly. And sure, it's from 2007... but I highly doubt the situation has changed...