Correct me if I'm wrong here: MyTaxi and Gett are solely for black cabs? This makes them a lot more expensive than private hire. Their UX is also slightly behind Uber's offering.
Addison Lee are also overpriced, and are not a company I'm willing to support due to their director's comments about cyclists and public transport.
One service you haven't mentioned here is Green Tomato, which have a tolerable app, but seem to have so few cars on the road that the wait time for a car is often a lot higher than Uber. Potentially this could be an opportunity for them if Uber truly do disappear from London.