I see there is lots of confusion about this topic. Part of the problem is that you are talking "backspacing" which is a really bad term to use, because it is not accurate enough for the calculation we are wanting to make. The problem is that we really don't care too much about the backspacing, what we really want to know is the "Offset".
Offset is the distance between the centerline of the rim, and the mounting face.
The wheel bearing system was designed not with backspacing in mind, but rather offset. That is, the most important aspect in all this is the distance between the centerline of the tire and the wheel bearings. Common to popular belief, the wheel bearings are rarely centered in the wheel. This would be desirable, but it's practically impossible to achieve given all the things that have to fit between the wheel and the bearings (ie: hub, caliper, etc.)
So, the vehicles are designed with a given distance between the wheel centerline and the bearing. This does create a torsional load on the bearing, but it the whole system is engineered to withstand it. The problem occurs when you push the wheel center further out away from the bearing. This increases the torsional load on the bearing beyond the design parameters.
You'll quickly see now why talking about backspacing is bad: Lets say the vehicle is designed with a 4" backspacing on an 8" wheel. That would mean the mounting face is exactly centered in the wheel, the offset is zero. If one swapped to 6" wide wheels with a 3" backspacing, one might think they have changed the torsional load. In fact, they haven't. If a 6" wide wheel has a 3" backspacing, then it also has a zero offset. You therefore, have not changed the torsional load.
Now you should see why backspacing isn't the correct term to use, because it does not account for changing wheel widths. Offset does, it really tells us what we want to know: How far is the mounting face of the wheel from the center of the wheel, and that will determine how far the center of the load is from the center of the bearing.
It does not matter one bit if the total offset is achieved via wheel design, or with spacers. All the bearing cares about is how far the load is and thus how large is the torsional load.
That being said, if a vehicle is designed with hubcentric wheels, we do need to use properly designed spacers. On many cars, the vertical load of the wheel is carried not by the wheel studs, but rather by the "keying" of the machined hub shoulder into the wheel bore. The hub fits snugly in the wheel center bore, and the studs are really only being used to clamp the wheel to the hub, but not resist vertical loads.
I believe Rover wheels are NOT hubcentric. But I'm not sure yet. If the wheels are hubcentric, you need to use spacers which have a machined bore on the inner face to fit with the hub shoulder, and then the spacers themselves need to have a machined shoulder on the outboard face.
If your vehicle is designed to be hubcentric and you lose that relationship, you are putting the vertical cyclical (as the tire rolls along) loading on studs which are really only designed for clamping. This could eventually fatigue them, and lead to failure. This is one part of the problem that causes the myth that "spacers are bad".
The other reason is that spacers are commonly used to push wheels outward, increasing the track width, and this increases the torsional loads on the bearings, leading to failure.
If you are using spacers to adapt wheels with too great of a positive offset, and you have the hubcentric design built into the spacers (or not, if not required) then there's nothing wrong with using spacers.
I currently use a thin 4mm spacer on my track car so that the inside of the wheel spokes clear my 4 piston brakes. I've never had a problem.
