designed to the same principles that led to the demise of the British aircraft industry (ie use as little metal as possible and spend as little on development and testing as you can get away with).
If, by "using as little metal as possible", you mean "lightweight metallic structures":
Considered changing to a different industry?
Otherwise -
If, by "using as little metal as possible", you mean "composite structures":
See Clandestino's response.
Use of composite structure in the DHC-8-400 is generally limited to:
Leading edges.
Fairings.
Some control surfaces.
Landing gear doors.
Hatches.
Interior panels.
In this regard, it's no big change from the DHC-8-100, 200 or 300 (same Type Cert), and ofr that matter, no different from design practises employed by most manufacturers of similar-sized aircraft.
Again, considered changing to a different industry?
In any case, what you call "the demise of the British aircraft industry", I would call the "rationalisation". Britain still has a strong involvement in aerospace through BAE Systems and Airbus, even though some people might say otherwise. The number of companies has reduced through mergers and takeovers, however the same process has taken place all over the world.