I'm not sure that's a correct analysis. As far as I know, anyone who can pass the knowledge test can get a cab license, so it';s not like there's a quota on the number of cab drivers or something, as you find with many US taxi monopolies (and which I have no time for).
Having service standards isn't exactly the same as protectionism. For example, it's easy to see the problems that would arise if someone just bought a bus, painted it in the same livery as public transit buses, and started picking up passengers on a popular route. I find the Black Cab situation interesting for similar reasons - they're not regular cars with a taxi sign on the side, but special-purpose vehicles, capable of accommodating a wheelchair, with the ability to use transit lanes, and so on. I think their beef with the regulator seemingly dumping licensing requirements on a whim is quite understandable from an economic point of view.
It's worth remembering that regulations governing cabs in London are not the result of regulatory capture by market incumbents, but were drafted centuries ago to deal with a tragedy of the commons problem, ie too many horse-drawn carriages causing traffic problems. The distinctive design of the modern cab (which are made by two firms, a British one that just does taxis, and also by Mercedes-Benz) meets standards set many years ago for ground clearance, disability access, and turning radius. Taxi firms have often asked for conditions to be relaxed over the years (especially the turning circle requirement, which allows cabs to do u-turns in places that would be impossible for other vehicles) but they remain pretty stringent.