SWA cables are great and should be used more often instead of surfix/twin & earth.
The issue with using the armour as an earth is that it requires that a few strands of the armour need to be fed through the CCG gland and these need to be terminated.
The glands often come adrift from the armour when under strain.
As far as the legalities are concerned I’m not sure. My house has an earth spike at the old meter box but it’s now disconnected… Is this where the earth should be terminated? I don’t see the utility providing an earth…