There are helper springs that are designed for things like towing. They're usually a rubber ring that fits between the coils to make the spring stiffer. They're also quite good at popping out and bending the damper if it's fitted inside the spring. They're not recommended by the RAC, AA, or caravan club and are potentially dodgy from an insurance point of view.
The helper springs being talked about are like this;
They're pretty soft and designed go coil bound with the vehicle at rest.
To fit them you also need a collar
By the time you've bought the helper springs and the collars, you could have just bought a set of softer springs that restored the original ride height.
Here's an old thread from this very forum!
http://www.madabout-kitcars.com/foru...ead.php?t=3312