[Treeve]

The Jag rear end is different to most double A-arm setups as the driveshaft is used to carry loads and support the upright. The lower arm plays an equally critical role in supporting the upright. The inboard brakes are where they are to limit torsional loads at the upright, and move the force paths back to the inside of the diff carrier. The trailing arms are free moving so do nothing to prevent torsion of the lower arm. Finally, you have symmetrical springs in front and behind the lower arm to evenly distribute any loads and prevent torsional forces from wheel bump on the lower arm.
By replacing this symmetrical layout with a single spring, every single bump / drool action puts a torsional load through the lower arm and bushes at each end. There is minimal resistance to this as the entire design philosophy of this rear end was that with a dual spring setup and internal brakes there wouldn’t be any... and if there were, the symmetrical springs would act to resist them. By having a heavier single spring you remove the torsional resistance of the lower arm and in fact cause torsion over every bump.
Edit to add: the later single spring jag rears with outboard brakes have a different designed lower arm and different spring geometry to change the force paths again to minimise this.
Treeve