I'm gonna be a right rebel here...
THE METRO!
OK, it had some fundimental flaws but... it was the UK's top selling motor of the year more than once, it allowed development of the k-series engines, kept the MG brand alive during the 80's after the MGB demise, spawned a rally program with the 6R4 (yes, I know it wasnt really the same car...)!
This small car which was supposed to be a spiritual sucessor to the Mini, kept going 18 whole years alongside it and was just as British too!
All of this helped keep Rover alive and kicking and allowed them to keep the cash flowing in to work on the Honda partnership work.
It also spawned the development of what became the mk3 200.
Any doubts about the popularity of the Metro - Watch and see how well its own Spiritual Sucessor the CITYROVER sells later this year!
Although the downside is that its not a British design or even made in the UK

But thats a different debate altogether
