The minicab firms which have fleet insurance do so because
they provide the cars. They charge the drivers for the insurance
just as they charge for the vehicles. I'm not aware of any minicab
firm having fleet insurance for drivers who provide their own vehicles.
I don't see how it could be legal anyway as the minicab firm would
not have an insurable interest.

The obligation on such minicab firms is to check that their drivers
have valid insurance and to refuse to give them any work until they
have proof of valid insurance. This is where Uber seem to be grossly
at fault. If the police have found that some Uber drivers do not have
insurance, TfL now have a compelling reason to revoke Uber's license
to operate as a minicab firm in London. TfL should also, of course,
revoke the drivers' licences.