You main earth terminal at the consumer unit is connected to earth via an earth stake. The return to the sub station is basically through the "earth" to where the DNO have place their own stake.
Come on fella, use your brain, how can the earth fault path use the neutral????
Go draw out a circuit including the supply transformer and you'll see where it goes.
I suggest that you do no more electrical work (practical or theory) until you understand what the following do:
Phase (aka line)
Neutral
Earth
Also read this. It gives you the info you need including detail on TT. Come back after you've thought about it, if you have any questions.
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