The funny thing is, British cars are still pretty damn poor. Ever looked at the panel gaps in new Rovers, or the way they shed trim? My flatmate is still trying to sort out the interior of his new Mini, which is falling apart (built in the UK). I had a 1974 Toyota Crown, bought it with 120,000 miles on the clock and 22 years old. OHC, smooth auto box, electrics all round, power steering, beautiful fit and finish. In fact, better built than many new Brit cars (I wonder if my neighbour has noticed the orange-peel effect on his XK8?) I still remember the crest fallen look on my father's face as his 1970s Hillman broke down, again, and again...
I think those who rave about what a good car the SD1 was probably owned one after its first flush of youth- that is, after all the defects had been ironed out by the first owner. It is a well documented fact though that sd1 built quality was pretty dire even by the lower standards of the day. yet another "defeat snatched from the jaws of victory/own goal" by British leyland. They always seemed to come so close with car design, but always managed to let it down somehow.
I don't think they would have got better with age because most of the earlier ones were dumped on the second hand car market fairly rapidly and flogged on for almost nothing, and someone who was buying a car for that sort of money wouldn't be likely to take care of it.
I think the mechanical side of things would have been okay if assembled correctly. I think BL bought in the automatic gearbox and the engine was a General Motors unit to which BL bought the production rights. In fact I think many people used to buy old SD1's solely to pinch the engine from!
The interior quality was also terrible - particularly the plastics (i.e. everything rattled and squeaked and it was very easy to strip the threads in the plastic mouldings) and the electrics were very prone to failure.
The Crown was five years old when I traded it in for the SD1 and it didn't have any rust on it that I remember, and the only failure it suffered from was the central locking (twice), a known fault which Toyota fixed. I think it suffered from having very few dealers, people weren't keen on the rather odd styling, and it wasn't as good dynamically as the competition (it wasn't exactly fast).
Perhaps ironically I gather more recent BL products (or Austin-Rover or Rover Group or whatever they chose to call themselves) have often had Honda engines.
I agree, as a pampered. cherished show car an sd1 should be OK! I tried using one every day and its heartbraking to watch it visibly deteriorate in a short space of time.