-
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.
I’m interested to hear that skateboarding in the road is illegal - it used to bring joy to my heart to see people skating up the OKR and it always struck me as a good thing, partly as a reminder that there are many ways to use roads.