The campsite electric supply will dictate what you can do with it,some have trips as low as 5 amps so you need low amp utensils,or you keep having to pay 5 pound reconnection charges,these are the extreme cases.You can get lengths of cable at any camping,caravan or Halfords shop,with the couplings you will need,however be prepared for the odd site with 13 amp plug outlets,there is one near Nottingham.This is why you need the normal lead with several differing adaptors.You can only load your supply with appliances that will cope,electric kettles,tv,s,slow cookers etc.It is a very sharp learning curve,enjoy yourself and good luck.