Well, you could get a Caterham or some other low-volume car.
K-series engine? Nope, that was developed by Honda. Zetec? Probably in Germany. Never mind, stick with a good old British V8, the Rover 3.5l... except that was designed by Buick.
If you can't buy a British car, blame the union reps of the 1970s who would call a walk out for every stupid little thing. The motor industry was booming until the unionised walk-out mentality set in.
Slogger, is your bike a Triumph by any chance?
It is not possible to buy everything from one nation now. Electronics will be made in Malaysia, Taiwan etc. Many computer hardware manufacturers have a ".com.tw" web address for instance.
The nearest you could do would be "buy EU". But I doubt someone intent on buying British would be happy to buy French, Italian, German etc. I would LOVE to buy British, but unfortunately it isn't possible, and won't be possible without a significant drop in cost of living. British car-builders aren't going to be happy with 4p and a bowl of rice for a day's work now, are they? But that's what you can pay people in other parts of the world to do the work. The best hope they have is fully-automated production lines.
Personally, I think it is a credit to Rover that they managed to keep it going so long despite being bent over a barrel by BMW from day one.