Perhaps im wrong here but to me, the older examples were marketed as 'toys' and the newer examples are 'models'. Britains were always regarded as a play thing whereas the newer tractors shown above are really shelf models and as such, have much more detail are are finished to a much higher standard. The comparative cost of these examples too would a reflection of this. A modern tractor such as the Fendt, Mercedes, Valtra would have a much higher price because of this also, compared to the more affordable Britains model offered for a few pounds upon its release, back in the day.
Just my 2c!