TFL are very clear that "Transport for London (TfL) licenses and regulates taxi and private hire services in London. As part of this, we set the maximum fares that taxis (black cabs) can charge"
It's not unfeasible that less could be charged to increase competition. After all, black cabs can be cheaper with some apps anyway.
Thanks Vanilla, I can only assume from your statement that TFL say that they set the maximum fares that Black Cabs can charge, that TFL mean that THEY REGULATE, and CHECK that the meter is calibrated correctly to run at Rates, 1, 2, and 3, and cannot exceed those Rates.
Whenever there is a change in the metered rate, this is done by the companies that supply and service the meter, it involves removing the seal, adjusting the meter electronically, then resealing the meter, all overseen and checked by a service technician, employed by the meter supplier, and authorised by TFL to inspect the finished article.
Then TFL will periodically stop and check taxis to to ensure that the meter is calibrated correctly, if it isn't, TFL will disable it, until it's recalibrated, resealed and checked, you can't work without a meter, so you're out of work until the meter is fixed, plus you'll be looking at a prosecution.
If there is a way of overriding the meter calibration, I'm unaware of it, and a driver would be pretty dumb to try it, as the punishment is a bit heavier than a slap on the wrist I think.
I did hear, years ago, that taxi drivers in some Eastern European city, Prague?, Budapest?, had been found to have fitted smaller wheels to their cars, as their meters were calibrated to tick over according to how many revolutions the wheels made, ergo, smaller wheels, more revolutions.
I can't speak for every Black Cab driver in London, but personally, I couldn't be ***** with that.