Why not have a set of standard steel wheels and jaggy tyres for off-roading, and keep the 18 inchers for posing on tarmac?
Those very low profile tyres give Discos a rough harsh ride, and eventually it will knock seven bells out of everything else. Remember, your TYRES are your first SHOCK ABSORBERS, followed by the springs. If you use hard running tyres then from the wheel-bearings inwards, every part of the suspension will get battered that bit harder. I bet LandRover knew that when they chose the tyres sizes for Discoverys.
The hydraulic dampers only stop the car oscillating up and down on bumps - hence DAMPERS. They do NOT absorb SHOCKS. Indeed, on 'first bump' they make bumps worse!
We used to be able to get dampers (from Monro I think) for our rallying Minis, which were "easy down and stiff up", and on bumpy surfaces at speed they pumped the car up a couple of inches. I don't think any of mus go fast enough on the ruff-stuff to make that a viable idea.
CharlesY