The rentals go 15kph. That is hardly an issue. Bicycles and skateboards can go just as fast (even faster) and are just as dangerous. I've seen these in a few major cities in the US and abroad. The issue isn't really with the speed. It is with people dumping them wherever they want, not worrying about propping then up neatly out of the way or on the kickstand. Drunks operating them is also an issue...but this is an issue with any mode of transportation.
I was 2 weeks in London prior to retiring in the Philippines. Stayed at a hotel very near to Hyde Park. For those that have never been to London, it is a very walkable City. Traffic is extremely congested and about 5 times the Wife and I exited a Taxi as your caught up in traffic with the meter running. and you can be caught up 30 to 45 minutes trying to get 8 blocks away. London needs scooters and less cars. Generally I would sway away from riding anything other than a super-bike or car, but London "SO BEGS FOR LESS CARS".
Total agree "Just as DANGEROUS" One of my friends and I often walk the "Wirral Way" which is open to both walkers and cyclists. There is a campaign slogan "Think bike think speed" .....I now have made a safety waistcoat with "Think bike think DEAF" on it, the times we have nearly been run over due to cyclists who dont slow or just assume you know they are coming from behind you! It's true heavy congestion in London especially but on the occasions I visit London I stick to the tourist red buses on/off circular well worth the money and can travel all day 24 hrs one off payment.