Ethics/legality of lane straddling when approaching r'abouts

On my route into London at Deptford there are signs telling lorries to straddle both lanes and that’s for an s bend. Of course that doesn’t stop the muppets in their audi’s/mercs pulling along side you. But they are the same ones that will sit in the right hand lane at the next set of lights despite the signs saying the lanes merge straight after the lights.