Sub main earthing arrangement

Just after a little guidance on a sub main install as ive never installed one.

my only "issue"' is how to install the earthing as in good wroking practice. rough plan being meter tails->sub main->3core swa->consumer unit.
I plan to use the 3rd core as my earth (armour will be earthed too but im using the 3rd core as my primary main earth).

Would it be considered ok to run the 3rd core straight through the sub main and connect that to the met, then run a 2nd flylead from the met back into the sub main to earth that, reason id like to do this is the earth bar inside the sub main looks like it will take a 10mm2 cable max. Alternatively is it considered ok to strip the 3rd core earth and split it across 2 screw terminals on the earth bar? Then ill run another earth from that to the met (This seems a bit yeehaw, plus im sure there is a reg regarding splitting the main earth between supply and consumer unit

ive read in the and cant see why my first option would flout the regs but im not perfect.

Thanks