Either configuration of the whole installation for two more cu. I would explain to the customer. It can not be done.
Then it will require to have the whole house CU on the ground floor while I wire the cables from the first floor and loft down to itEither configuration of the whole installation for two more cu. I would explain to the customer. It can not be done.
The gateway needed is 1 as per the diagram . They want non backupYou will need to know from whoever is installing the Powerwall where they are locating the both the Powerwall unit and Backup Gateway(s), essentially just a changeover relay / contactor and whether they can manage with 1 Gateway or they need 2.
That'll determine how to run the cables to each C.U., you can't plan anything without knowing that.
Split load CU's should work
Reply to the thread, titled "Rewiring a 5-Bedroom House with Powerwall 3 Integration" which is posted in UK Electrical Forum on Electricians Forums.