In the UK, the government defines classic as pre-1973 and therefore, you do not have to pay roadtax on these vehicles (you do still have to have an MOT every year).
As for the classic range of Morgan 4-wheeled cars - Morgan call the range Classic; however, as noted above, the name traditional cars seems to be used by many enthusiasts (possibly in more recent years as a way to define the difference between the Aero range and the non-Aero Morgans).
Since the birth of the Aero 8, Morgan have slowly filtered some of the technology into the Classic range (Superform panels as an example); therefore, you could say that a true classic Morgan is a Morgan built up until the point where the company introduced modern production parts. However, imho, this would be ignoring the fact that over time Morgans have continually improved and embraced new technology.
I recently had the pleasure of a drive in a 1954 Plus 4 DHC and trust me, it is light years away from a 2010 Plus 4 (and that is before we even get onto the AeroSS!).
To my eyes, the Classic range has such appeal because it looks like a classic car but doesn't alienate the owner by suffering from the myriad of problems that many associate with classic car ownership (rust, unreliability, unrefined drive, no creature comforts, etc); however, this does not make the Classic Morgan a classic, it simply means they mirror the style of one.
Ruby, it is an interesting question and I would say we all need to draw a line in the sand wherever we see fit. For my part, I stand where I always have: if it has a Morgan badge, it is a Morgan and I could care less what other classification someone else puts on it!