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.

Try this BG Metal Shower Consumer Unit IP65 2 Way.

I used one temporarily to replace a very tired outbuilding DB. (Tired DB, not tired outbuilding!) They are built like a brick dunny.

An alternative solution would be to put an ordinary DB in a meter cabinet or something similar.

Thanks Chris, I had considered a meter cabinet but they appear to be around £40-50 so it would seem the IP44/65 option is lower cost.

Give a thought to maintaining the IP rating when you cut holes in it for cable entry - flat T&E cables especially aren't great with normal cable glands that are designed for circular cables. Special glands for T&E are available, but they take a little hunting out. Outdoors also be conscious of internal condensation and insect ingress.

   - Andy.

Thanks Andy, have run armoured cable underground so will be SWA glands into consumer unit, good tips on condensation and insects

do you remember how long the busbar is? it states 2 way but looks like 3 usable ways? i used them once but cant remember.

Those waterproofing compression glands that grip T and E properly are sold by TLC and others

https://www.tlc-direct.co.uk/Main_Index/Cable_Accessories_Index/Gland_Compression_Nylon/index.html

No commercial connection other than I have an account with them.

Not as cheap as the round ones, but very good in cases where a blind grommet with a knife slash is not really good enough.

On boxes where you may struggle to get a good earth on SWA glands with banjos, and do not want to compromise the corrosion situation, the "Piranha" nuts are a good alternative.

https://www.tlc-direct.co.uk/Products/PRPEN20.html

Again useful in damp outbuildings or plastic boxes and so on and on metal conduit.

Mike.

The CU/DB (consumer Unit / Distribution Board) should use Main Switch & RCBO and not MCB.  Consider the cable from the house to the shed.  SWA (Steel Wire Armoured) is normally used, worth considering using something like Docaster EV ultra it can deliver power a data via cat 6 in one neat bundle to the shed.  Use it for WiFi.

Depth of cable if buried at least 500mm below glound will add 1 meter of length to your cable.  Then do cable calculation with volt drop to confirm.  To be honest this is the role of the designer which will probably be your electrician.