A single/triple 13 amp socket(s), a distribution box with a transparent window housing a 10amp double pole MCB and 25amp RCD. This should be in an integral housing and physically protected to a minimum of IP44.
This in turn is connected to cable with a BS EN 60309-2 plug (Commando Type) at the opposite end.
These are sold complete at most reputable camping outlets. They are available with a single socket outlet as shown above or three socket outlets as shown below.
You cannot sensibly expect to pay less than £50.00 for the single unit, more for the triple unit, so if anything is selling for less than this, treat it with caution.