Actually it IS, as long as the cable is thick enough and runs to the Main Earthing Terminal near the incoming supply or in the consumer unit.
It should be 10mm² today but may be less if installed a while ago.
The regulation actually states it should be connected to the pipe on the other side of the meter but this is not the best place for effective bonding - not earthing.
You could connect the other cable to this pipe for the best of both worlds - if they run to the correct place.
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