Not having the ARB fitted is not an MOT issue - not fitted not tesable. Also likely not an insurance one eithee considering not all Defenders came with one. Its also not really possible to know which came with them and which didnt. My 110 is an XS so should have them but does not.
As for dangerous dont be ridiculous. My caged 90 had no ARBs, on standard suspension, sure a bit wallowly but not dangerous.
My 110 has no ARBs, full cage and lifted but OME +2 springs with Koni HT Raid dampers. Also not worried about coming into corners at speed, even done some mild drifing... The ride is brilliant, better than a lot of Ders with ARBs ive driven! So not dangerous no.
I also do 350 miles a week to work, driven to Spain, France, Wales, Andorra and the Pyrenees. Never had a problem with handling...
See how it drives without it, if youre happy save your money. If not new bushes (super pro are brilliant) and new bolts.