Cougar, your thinking along the right lines.
Roof top tents are far from a new idea and very popular with overland tours of Africa, there a large portion of the small wildlife can be unpleasant and quite toxic to your health, then you have the wildlife that views you as an English takeaway, although if a lion/tiger/panther/meateater with big teeth really wants to eat you it will investigate a top tent.
Then we have the big heavy weights, cattle/elephants/rinho/hippos a stampede through a camp or just a hippo coming to have a wander through late at night…..tent on the ground is no protection.
In reality for UK, if it’s very wet, possible minor flooding or you pitch up in a field full of curious cows then they will be of use.
Another factor is freeing up vehicle space, you can carry lots of weight on the roof and sleep inside the vehicle, this makes a top heavy vehicle and prone to falling over when serious offroading.
Carrying all the water and spare fuel low down is more stable and secure when in remote locations.
Look for the Pinzgauer and Steyr overlanders 🙂