From the DEMO Europe coverage I've been contributing to Mobile Industry Review:
Wheely is a cab booking service that will look familiar to users of Hailo, Uber and Adisson Lee’s services, offering booking apps on iOS, Android, Windows Phone and web. The service currently operates in Moscow with a fleet of black Mercedes cars and – after a trial in London in 2012 – is due to re-launch there in mid-June, offering lower costs (30% less than black cabs), hybrid cars and free 4G-powered WiFi.
I've been using the service extensively whilst in Moscow (disclosure: funded by our hosts) and it works well. The WiFi, especially, has been a great way to work whilst avoiding roaming costs so could be useful for visitors to London who pre-book.
Pick-up times will be longer than Hailo or GetTaxi who use black cabs, but lower prices for longer journeys offset that. Addressing under-supply of cars at peak times is also a shrewd move - it's one of the most frequent complaints I've heard about Uber.