As an electrician, I agree with Ozzie to a certain extent. Granny chargers are used on a ‘at your own risk’ basis. It doesn’t have earth protection because, basically, it doesn’t really help to have one. The reason being that it would be connecting to your house earth and your car is probably not in your house. The time it would take for the earth current to go through the cable, around your ring circuits and to your distribution unit would be an age compared to through you and ground.
Plus you have to remember what would be needed to cause a fault enough that the body of the car would become live. It would take a catastrophic failure of the charging unit inside the car and it’s protective circuits AND the granny charger at the same time. Whilst you are touching it. For the split millisecond before your RCD detects it. The PEN or PME only add another level of protection. This extra level would be a lot more recommended for 3phase chargers, the sort required for 7kW chargers. Up to 3kW on single phase 240v, it shouldn’t be as big a risk.
The main reason they would say it is for emergency only is because the circuits used in the MG supplied unit are not up to the same standard as a mounted one and is subjected to all sorts of potential mistreatment. Cables getting bent, stressed, cut or otherwise damaged. Like a laptop power supply, if it’s well looked after it will serve you fine for many years, day in, day out. However, if you are constantly getting it in and out of its bag, folding and straightening it, you are doing ‘untold’ damage to it and risk causing a failure. But, fortunately, the device or the car will pick up on it and prevent it from working before it causes you or anyone else any harm.
If you are running a line outside and not just to a garage, I would recommend using 6mm² steel wire armoured rather than standard 6mm twin and earth. It will serve you better in the long run even though a little harder to work with.