It should be said as well, the army don't need land rovers much anymore, they came to the end of their usefulness on foreign soil when IED's and explosives became more widespread, modern wars need specialist vehicles which are built for war, that takes a lot of investment as others have said, for limited return if any. Fitting armour to a defender just wont cut it anymore, and trying to design something from scratch, doesnt in my mind make much sense for land rover.
Below is the foxhound which replaced the defender, think you can see the similarities but the mine protection underneath is next level -
its definitely doable for land rover, but is it worth it.. i dont know