I skateboard to/from work every day except when there's snow, and have done so for years in several cities around Europe, including London. And let me tell you: major cities, with London as an extreme example, will
never be skateboard friendly. I will pay for my skateboarding habit with my knees when I get old. Some places have even started adding these ridges in the sidewalk that are there to help blind people find their way to bus stops, pedestrian crossings etc, and those are absolute killers.
No, even if adoption of various segway-clones suddenly flies through the roof, the roads won't be good in the growth phase so everyone will buy the models with big air-filled wheels, and thus there's no incentive for improving sidewalks.