The 602 is not hard to find and can be purchased in good condition with a box for around £50
Vectis Auctions recently sold a prototype, made in 1975, for this model. The key differences are the colour scheme and it has cast metal wheels rather than the plastic ones on the original model. Not in very good condition but it still went for £130
The all plastic yellow version below is also a prototype, It was not a prototype for the metal model which went into production but for a range of all plastic versions of existing Dinky models that the company believed would be much cheaper to produce. The range never went ahead. The last version Vectis sold went for £600.