SAFETY WARNING !
Do NOT modify the mounting bracket, as that annoying spring-loaded spigot is a Safety quick release mechanism.
Routine maintenance is also a Safety requirement.
As someone suggested, Land Rover probably used a spring fastening mechanism, as opposed to a solid screw up [intentional pun], as you want that big arm to break away if your head hits it in a crash. For this reason, it's important that the spring-loaded mechanism works freely. If it has seized, then it'll take more force to break it away. For this reason alone, routine maintenance -- freeing up and lubrication -- of the quick release mechanism is essential.
(And if you've modified the mount so it's permanently screwed in place... Well, I think the implications speak for themselves. You need to remove those screws and do some maintenance work to free up that quick release mechanism for your, and your passenger's, safety.)
Who'd have thought it?
Rick