I don't think this has been thought through enough - it seems to me a few incorrectly installed consumer units have caught fire so the IET have tried to legislate against incompetence by bringing in this reg, with little apparent consideration for the benefits of a correctly installed plastic CU over a correctly installed metal one, such as electrical insulation and resistance to corrosion.
If they're going to insist on metal CUs just in case they're installed incorrectly (to offer increased protection from fire but decreased protection from electric shock in the place you need it most, ie before the protective device) you have to ask "what's next?" Will we see all plastic accessories 'banned' just in case they're installed incorrectly and could start a fire?