In London, I’ve also used the myTaxi app to get a black cab. I have no idea which is better, perhaps locals can help?
The taxi drivers I talked to said they used Gett and Hailo, which seem pretty much indistinguishable.
For the most part, it's faster and easier to use your Oyster card on the underground or a bus, but taxis are useful if you have trouble walking or have a lot of stuff.
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