I think 'Most' is a huge overstatement, the last house I lived in without it was a student dive in 1996.
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...