No difinitive answer. Usualy "zero" as engine components have to heat up regardless if it is needed for motive power or not. Without internal combustion engine to keep main battery charged the battery cruising range is only a couple of miles.
I don't have the exact numbers in memory, but ordinarily it is about 40mph (with v---e---r---y slow accelaration), but a lower speed (perhaps 30) when in EV mode via a switch. I'm sure I'll get corrected on the exact values.
I find I just can't accelerate that slowly or traffic objects. I don't see anything over about 15 in traffic without using the EV switch.
As was mentioned, 42 is the absolute maximum. Higher than that, and the gas engine will start spinning. Note that this doesn't necessarily mean you will be using gas. If you don't need much power, the electric motor will spin the wheels AND the ICE, without any gas entering. I've seen this at speeds as high as 60 MPH. (I even accelerated on a freeway from 55 to 57 on level ground solely with electric power.)
If you apply the EV mod, then in EV mode it WILL start using gas when you exceed 35 MPH.
Those are the absolute limits. If it's twenty below zero, the ICE will come on as soon as you start the car, and stay on for quite a long time as it warms up. (And if you have the defogger on, you guarantee the ICE will stay running.)
On the classic, MG1 had an RPM limit of 6500. In the new version, that was upped to 10K. Still, the ECUs seem to make the ICE spin at 42 MPH from reported scans, even though it could safely go to 65 MPH before ICE spin would be needed to compensate.
Well, on the classic, the engine spins to prevent over-reving MG1, which would happen at 42MPH if ICE did not spin.
On the 2G, the rev limit is 10K, not 6.5K, so the car could be going faster before ICE would have to spin to prevent MG1 damage.
Yes, the drive belt does drive the mechanical water pump, but if ICE isn't fired, the coolant doesn't need to circulate, as there is no excess waste heat to get rid of. If you need cabin heat, there is an electric pump to provide circulation for the heater core.