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Yes, it's illegal, as with unmotorised scooters, to use them in the carriageway. In practice, I don't think it's a problem if people do it, and I can't remember ever hearing about a serious crash. It's one of those things that sorts itself out; those who don't have the skill to do it won't do it, and other road users tend to be careful around skateboarders, partly because it's so unusual.
Not wishing to speculate, I could imagine that the inspection cover may have hit the rim of the wheel because of the lack of air pressure, either throwing her off course or causing the tyre to explode. (I think the latter may be less likely.)
Either way, it serves to highlight one of the risks associated with these scooters, but there are many others.
It's infuriating that against expert advice the Government has caved in to lobbying and removed one of the few things the DfT has long got right. Micro-(auto-)mobility is going to have a terrible crash record particularly on London streets, with their sub-optimal carriageway conditions. One does hope that, as with (non-e-)skateboarding, the activity might not become as popular here as it is in European cities. (NB because of its lower speed I don't consider skateboarding in the carriageway, though illegal, a particularly hazardous thing to do; that's obviously different for e-skateboards.)
RIP Emily Hartridge, a senseless and tragic death.