question

Unfortunately, this is not as simple a question as it seems, so a simple answer is going to be difficult to provide because it is quite subjective.

The absolute simplest answer is that the heatsink of the MPPT needs to be connected to the same earth system that the rest of the equipment will be bonded to.
The wire size needs to be able to carry the full fault current until the circuit protection device opens the circuit.

In a purely DC "Extra Low Voltage" separate floating system with no earth reference then there is nothing to bond the heatsink to. If the highest risk is that a live conductor of a 12V system contacts the heatsink then the risk of an injurious electric shock is low.
A single solar panel is also below 48V, so also considered "Extra Low Voltage" in the UK. This will also likely be floating without any earth reference.

There are various earthing systems in use in the UK and the location of your system and the proximity of it to other systems needs to be considered. You shouldn't simply hammer an electrode stake into the ground in close proximity to another earth system that is currently in use.
If there is a house which is using a TN-C-S system close by and it's supply suffers a broken neutral then your earth stake and the system connected to it could have a high Voltage on it since the only return path for the TN-C-S system is now via the physical earth.
If your earth stake is far away from any other earth systems and the risk of penetrating any other services underground is nil, then you'd be able to create your own earth system.

When you upgrade your system to include an inverter at 230V I'd always urge you to consult your local electrician for the best course of action for your given situation.

For reference, Mr. Ward explains in more detail: https://youtu.be/mzCGE5wnWos