Notice 75 was replaced by CAP747 GR NO.17 several years ago and states the propeller should be overhauled:
Whichever occurs first of operating hours or calendar period as published by the propeller manufacturer.
So even if an aeroplane (with a VP prop) only did 100 hours over this period (typically 6 years ish), the propeller would need a full overhaul. For a low utilisation aeroplane this can be a significant cost per hour.
The Bulldog is a magnificant aeroplane, I've flown many (inc first solo on UAS) and maintained a fair few. However, having recently purchased an aeroplane (Bulldog was considered) I couldn't quite bring myself to spend my own money on one. Not a very efficient tourer/IFR trainer with all that built in headwind + only 2 seats (unless you get a Sweedish/Hungarain one), and local aeros jollys get boring eventually especially with £1.87 avgas.