Get your tame electrician to agree this, but the prohibition on plastic consumer units is to prevent fires inside domestic property. You may sensibly argue that if it is an a location which even if it burst into flames, the house would not be affected, that a plastic unit is perfectly safe. 
Personally I have a couple in damp garden-shed like places where a steel unit would be asking for trouble in terms of corrosion. - there are times when one has to be a bit pragmatic, and these are very reasons the plastic ones are still made and sold.

It is also possible to supply an outbuilding without a local dis board at all, as one circuit if the power demand is low, and a socket radial can have a fused spur as the light switch.

Also consider where RCD protection is, and how easy to reset if it pops off leaving you in the dark in the garage.

Mike.

Thanks Mike, I like pragmatic! The plastic consumer units are often labelled as garage/caravan units so it makes sense.

Thanks Mike, I like pragmatic! The plastic consumer units are often labelled as garage/caravan units so it makes sense.

I use plastic WISKA enclosures for outbuildings which are damp and not 100% weatherproof.and then populate them with whichever spare devices I have to hand, usually a RCD main switch and a couple of MCBs left over from a old consumer unit suffices well.

These things are bulletproof and waterproof.

www.sparkmonkey.co.uk/.../wiska-1306br-w-6-way-ip65-distribution-enclosure-with-din-rail