"The plastic fantastics were never designed for the training role"
I think almost all the aircraft mentioned above are metal.
If you want an example of a "modern Tupperware" aircraft designed for a training role then "The Diamond DA20 is a two-seat tricycle gear general aviation aircraft designed for flight training" would seem to prove you wrong.
"Add an extra £15 -or more to the hourly student rate"
The last time I looked I could hire a Eurostar cheaper than a C152 (both local to me).
Q What is the highest single cost in operating a training aircraft? A Fuel. We have 50 year old aircraft used in the training role because of product liability laws in the USA not because the 1950's aircraft are perfect and will never be bettered.
Rod1