You can use either galvanised steel boxes, or plastic boxes, either with an outdoor weatherproof rating (IP 65) You will need to use watertight glands to seal the cable entry points to the boxes. There are special glands to make off SWA (steel wire armoured) cable into the boxes. If you are running just single phase then a three core SWA cable will do the job - the gauge depends on the load to be supplied and the length of the cable run. Connect the steel armour to the earth conductor at one end of each length of cable. Also use a combined RCD/MCB rated to suit the cable size and expected load. You can put this in your consumer unit, if you are bringing the new cables back there, or if you are using an existing supply, e.g. breaking into an existing ring main, then put it in another IP65 box on the wall. You can get ones with a hinged cover and a DIN rail to mount the MCB/RCD. You local electrical wholesaler will advise. SWA cable is not easy to work with, but you can bury it in the ground. If you can clip the cable to a wall where it won't get damaged, then you can probably use an outdoor cable like NYMJ, which is much easier to terminate. Good Luck
