It’s not legal to drive a non-road-legal vehicle on the roads. True.
However, if I wanted to drive my tractor up to London (I mean, I wouldn’t, the red diesel cost would be heinous) I am perfectly entitled to. That would arguably be a ludicrous thing to do, but I can do it. The law permits that.
No one is saying we have the right to drive whatever we want, however we want. But driving an SUV in inner cities is perfectly legal. And rightly so. We don’t live in bubbles, people use vehicles to travel, and as such traverse areas. I drive 70 miles into London. Going from hill farm, to country lanes, to B roads, to A roads, to motorways, to A roads, to city streets. I do that in one vehicle.
I’m not sure what your proposal would be? Perhaps change vehicles at each stage, whittling down body and engine size as I near the centre of London?
Either way, it’s it going to happen. People are allowed to drive anything they wish within the confines of hill the law. And SUVs are very much legal.