Finally have some time to write a reply.
I am not
an electrician and have limited experience with UK wiring. This post is not installation advice. I welcome any corrections or suggestions.
The recommended earthing system for an outdoor charge pod is TT. I hadn't seen this grounding system in the US.
In US houses, the ground is connected to the neutral wire coming from the utility service connection. Many American houses do not have a ground rod at all (yikes!). If there is a ground rod, it is directly connected to the neutral line. Optionally there is a lightning suppressor connected between the hot lines and your house ground. Damage from lightning strikes is common as most houses don't have a ground rod or suppressor. Go figure.
TT stands for Twin Earth. There are two separate ground connections. One ground is back in the utility system somewhere. It is probably at the transformer back at the street level. This first ground is tied to neutral. The second ground is at the Charge Pod.
Here is a diagram google found:
The silver shaded box is the charge pod and your car. Normally power flows from the Line (L) through the load (your Zoe) and back out the Neutral.
The ground connection is for protection only. Normally there isn't any current flowing the ground. Under normal circumstances the ground doesn't do anything.
If there is a catastrophic failure, like running over the charger lead with a lawn mower, the line could be connected to the protective ground conductor. In this case, current does flow in the ground wire. A protective device, called an RCD in the UK, monitors the ground wire. If it sees over 30mA of current, the RCD disconnects the circuit. UK regulations require this to happen with in 0.2 seconds.
If the impedance between the local ground connection and the one back at the utility pole is very high, not much current will flow in the ground (dashed line in the diagram) In this case, the RCD may not disconnect the load. If true, there could be dangerous voltage present somewhere.
I'm not exactly sure what the standard for a good ground impedance would be. 200 ohms or less seems reasonable to me. But don't take my guess as correct. In theory an impedance as high as 7000 ohms would still be enough to trip the RCD, but a connection that poor won't be reliable. And you want this to be reliable.
One suggestion: Don't cover the ground rod with cement or anything waterproof. To stay under 150, the soil will likely need to be damp. If it drys out and there is a cement covering, it will be difficult to get it wet.