Things that killed the U.K. mass-production airgun industry:

1. Nationalism. Assumption that the 1940s and beyond prejudice against foreign, esp German/Japanese products would continue forever.
2. Practicality. Being stuck with old machinery, personnel, organisation, the unions, etc.
3. Lack of government intervention/investment (though that hardly helped, say, the car industry).
4. Lack of vision/foresight. Airguns were dangerous toys for kids, or rat-shooters in barns.
5. General economics - eg the DM/Yen/£ exchange rate in the 70s and early 80s.