[greenbeast]

There are lockable water resisting  power supply cabinets that can be set on a compatible plinth or on concreted in posts or a fixed to a stand alone solid brick wall 1200 mm or so tall 9 built on a proper concrete foundation with a damp course .

TLC- direct    item number CM SB10010025 is an IP^% coated steel box a metre by a metre by 250 deep

GRP Feeder Pillars & Cabinets , sell a wall mountable external meter box for around £ 35 .

 How does that compare cost wise with you constructing a specific building ?
 
They will hold a meter and a small consumer unit plus a couple of sockets and switches .. often used as temporary supply boxes on small building plots where  an electric mixer and a kettle in a shed are going to be used during the construction period .

 For my farm supply cabinet I purchases one of the big double door-ed fibre glass telecom's cabinets that stood on a 200 mm plastic plinth as I had six heavy SWA cables going into it through the concreted in plinth via sealed up ducts .

I ran the steel wire armoured in ducting into it & from it to all the new buildings . The reason for the use of ducting was that for a few pounds more to me it was sensible  should I change use of the buildings & they needed more power I could then pull in a new heaver cable system .
 When we sold up it was one of the unique selling points we offered to potential buyers ..
ie. Electrics easily & cheaply up-datable due to all the underground cables being in 3 inch ducts

 Marcus aka MAB here on site may have even better ideas as he does electrics professionally .

I'm just doing this now for our new supply. We're getting 100A 3ph supply and so the required dimensions are 2x1x1m. At this size the GRP kiosks/cabinets are £1500+
The materials for a block building are in the region of £200-300 including concrete base