I rode in London, a lot. Fast, too. Couple of accidents, once slipped off a pedal it speed, ran over my foot, but stayed on (thanks, physics), once got too close too a rising curb and couldn't steer, hit a fence at speed, got deep cuts and lots off bruises, but bike took most off the damage. Have had accidents since, usually from heavy breaking, with complication leading to launching over the front. I cannot see how these boards could be safer, any dismount at equivalent speed would need considerable skill to not face plant, and any complications would just add energy too the impact. That or you could simply quail too stop in time and hit something (like a car door opening, or a truck pulling out, or that kid/dog that ran in front of you)

I've only crashed once in Copenhagen, when a tourist turned left directly from the right side of the lane.

Oh, and once when I was so drunk I fell off 2 seconds after getting on the bike.

In London it's necessary to plan a quieter route, using roads most people don't even realise exist. It can't be as nice as the Netherlands, but it can be OK.