I believe the car has to have been built before January 1973 to be qualified for tax exempt status. Also it seems a large part of why Series 2 and 2A's go for more than 3's is because they are all tax exempt.
as far as I'm aware it's pre 1/1/73 that are eligible for free road tax, this 25 year thing was stopped years ago. (probly in '98 as that's 25 years after '73)
dunno where you get this £203 reduced rate from it's £205 normally so 2quid is not much of a discount is it.