It gets asked a lot, this question.
No good answers yet.

I reckon it must involve the regard in which the craft was held when it was new.
If people thought it was a beauty, and leader in it's class, a real gem, then as an old lady it must be a Classic.

The parrallel can be drawn with cars. An E-Type, a Mini Cooper S, a Lagonda with a hand-built cabrio body, an MGTD, A Triumph 1800 Roadster, a Rover 3 Litre. Many cars have a long history of being revered and sought after, or just plain held in esteem and are Classics now.

An A40 Farina, a Morris 1000, a Ford Cortina?

Certainly not Classics.