There is no need to replace existing Consumer Units unless defective or otherwise unsuitable for continued use, this being said any Consumer Unit that does not offer RCD protection should be upgraded.

In January 2015, IEE Wiring Regulations (BS7671) Amendment 3 was published. A major change in Amendment 3 concerned Consumer Units. New regulation 401.1.201 states that in Domestic environments, Consumer Units and similar Switch Gear must be constructed from non-combustible material

You do not need a Metal Consumer Unit, unless the existing Consumer Unit is being replaced / Upgraded you can still make alterations to existing wiring circuits using the old Consumer Unit unless new circuits are added or the property requires a Re-Wire.

Residual Current Device (RCD) is a must have safety device which switches electricity off in a fraction of a second should there be a fault within a circuit under RCD protection