I'd personally advise against running a disperate track width -- your rear being wider than your front, if only by 20-50mm or so, might have some unexpected consequences when it comes to traction control, torque vectoring by braking, and other stability management calculations.
I might be wrong; I've not played w/ spacers myself, and have no idea how touchy the stability management is w/ these vehicles. If you can get away with it, the bonus would be that you're not screwing w/ the steering geometry. I guess that's mitigated though since cornering performance will apparently take a hit (the smaller the rear spacers, the smaller the impact I'd gather), but then again I'll bet you're not autocrossing your Rover. `) Your turning radius will suffer to some degree as well.
If you do wind up testing the waters, make sure you've accounted for (i.e. restored) the wheel stud threads you'll lose inside the spacer. In my bimmer days, this was easy since those vehicles used wheel bolts that'd thread into the rotor assy, so spacers would just come w/ longer wheel bolts. I don't know how it'd work w/ standard lug nuts but I'm sure it's not too difficult. Just don't overlook that part. ;-)