Use of plastic backboxes rather than metal, use of multistranded rather than solid core cables for power/lighting

I am undertaking a complete rewiring project in Malta. The local electrical regulations are based on the latest IEE Wiring Regs, but there are few unexpected practices:

1. Plastic backboxes are used rather than metal. This is because they are chased into limestone blocks, not brick, so metal backboxes tend to corrode.
2. Single core cables are run inside plastic conduit rather than using twin core and earth in a PVC jacket. The cables are also multistranded rather than solid core, though for some terminations the wire is capped and crimped to give a solid core like end

I want to follow local practices as far as possible, but would be grateful for any advice about the above. What changes, if any, should I ask for? What are the disadvantages/ advantages of the above?