One thing to point out is that this is not run by the company that runs Uber, it is a private effort using a similar name.
To quote the beeb article...
Despite its name, Skyuber has no connection to the popular taxi-hailing Uber app that allows users to book a car and driver. And the founders say they don't want to compete with private jet companies and low cost airlines.
Based on the CAA guidance on advertising flights for cost sharing which I posted above I can't see how this is in anyway legal. Whilst it is legal to advertise a flight for cost sharing to other members in your flying club, it is not legal to do the same to members of the public on Gumtree, how is this different?