There was a few threads on some of the LR forums here about fitting a side mounted spare.
General concensus was that "it would be illegal" (for UK road regulations) but no-one was 100% certain on it.
although their vehicles have to be road worthy, the Military have special dispensation for some of their kit (like driving with a .5 cal machine gun etc!) and no-one really know whether the side mounted spare falls under this category.
You sometimes see these mounts on ebay, like this one!
, it bolts onto the wolf rollcage, so you'd have to have a play with whatever kind of roof you have (the aluminium side panels would be no-where near strong enough to support the weight of a wheel!)
SF WMIK rovers sometimes have 3 spares, one on the bonnet and one either side with these mounts.
The wolf's also have these "tufflift" spare wheel pullies
to help the smaller bodied killing machine to haul the wheel up onto the mount, and save the wrath of health & safety and a workplace injury