It is a mistake to think in terms of military and civvie here, they are imperial or metric. When you are deciding which tyres to use you need to decide what you expect of them.
An imperial sized tyre has a 100% sidewall to tread ratio so you get a tall tyre for the width, this has advantages off road like clearance, cutting through mud or snow and having a decent steering lock.
On the road you will have a smaller contact area on the road than the same height metric tyre, this makes steering slightly lighter and rolling resistance slightly less, in theory this would give better economy but probably not noticable in practice. On the downside the tall sidewall gives less stability when cornering and the narrow contact area gives less grip in the wet.
Metric tyres have the lower profile which helps on the road but off road the wider lower tyres don't work as well. As most people spend most of there time on the road metric has become the standard fitment, however, Land Rover still list 7.50x16 as an option on new Defenders and they are available if you look for them.
A last point, when you get your tyre the rim it is mounted on affects its inflated shape. For example, the swb landrover rim is narrower than the lwb landrover rim so the same tyre would appear wider on the wider rim.
http://www.landrover.com/gb/en/Vehicles/Defender07/Specifications/Defender_features_and_options.htm?route=_gb_en_Vehicles_overview@__link__ContextLink_4_4#tableheading1
http://www.colwaytyres.co.uk/ctraxat.htm