I'f you are referring to a modest length flex then 1mm² should be adequate.
If it's going to be buried in the wall , under insulation etc it may need to be thicker.
It's one that already installed. I'm questioning the installation as opposed to MIs. Must of been done by and electrician as the ends of the cables are crimped if that's what you call it. And has a special earth connector on the earth.
Only if it's far too thin and overheated to the point some of the plastic insulation has melted allowing wires to make contact.
But that is highly unlikely; it would have been smelly, it would be badly misshapen and very lumpy.
A 0.75mm² cable rated at 6A would probably not be affected by running at 8A and indeed is commonly fitted to 2KW fan heaters, as an example:
The chances of anything thinner being used is in my opinion so unlikely that I'd not consider it.
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