It will not be 4mm it will be an imperial size which I think is just bigger than 6mm, anyway it is not part of the instalation it is part of the DNO supply
Hopefully using a 100mA S type RCD as main switch so that would be enough protection and just use a MCB if not as said previously it's own dedicated RCBO.
As the the Honeywell nedded a neutral it could be the blue is the neutral and the switched live is the green/yellow besdt to check at the other end first though.
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