As a cyclist, I can tell you that the number of times I see these taxis running on electric only is near zero.

Even if they theoretically get 80 miles of electric only range, and can be plugged in, the reality is the drivers force the engines to run to provide cab heating, avoid downtime while charging, and to avoid wear on the battery.

My guess is in the real world, even excluding manufacturing, these produce more CO2 per mile than the TX4.

That’s one of the known issues for hybrid vehicles, that they’re used as standard fuel vehicles.

One of the main reasons to go all electric, maybe there’s some design slight of hand that can be used to increase the time electric power is used.

Theoretically, even if never plugged in, they should get much better MPG due to the regenerative braking and the ability to always run the combustion engine at the most efficient revs (which for city driving should ~halve fuel consumption alone).

The reality is due to a series of poor design choices, that's rarely the case. Normally due to certifications on the braking system, the regen braking isn't actually hooked up to the brake pedal, so most braking is done with the regular friction brakes. The engine also doesn't normally run at the most efficient RPM because users don't want to hear the sound of the engine revving while they're stationary. Combine that with the fact there are efficiency losses in the battery and electric motors and the added weight of the battery, and you suddenly have a system worse than a regular car.

Are there any plans for certification's to catch up with regenerative braking ?

I've seen these in the Netherlands, nice vehicles. I think a retrofit kit for the old cabs would be nice. Keep the chassis, frame and wheels, swap the engine and tank for a full electric driveline. Given the scale it might be cheaper than new vehicles.