It depends on the location. At my favourite festival there is a creek with a spa in it where everybody swims; nobody uses soap in the creek. There is a cold shower located in a convenient part of the campsite for those who want to use soap and those who like warm water do as Sorcha suggests, well away from the creek.

The National Folk Festival in Canberra is held in what northern hemispehere people might recognise as an agricultural showground (if you're from the British Isles) or a state/county fairground (if you're from the US) with racetrack facilities. There are quite good toilets and showers but the influx of some thousands (many of whom camp there for the whole of Easter) overload the built facilities.

The NFF organisers have coped brilliantly with the extra load by hiring a set of four semi-trailers. The prime movers ("tractors" I think US residents call them) manouvre the trailers into position and then vacate the area, leaving the four trailers plugged into hydrant water supplies; all use water treatment to minimise total use. Two trailers contain showers (male and female) and two (ditto) contain toilets. Because they're still on their bogies (US people call them 18 wheelers but Australian "semis" use triaxled bogies, and are thus "22 wheelers") you have to climb a set of steps to use them, which is OK for those of us who don't get around in wheelchairs. Those of us who DO need wheelchairs are provided for by the already existing facilities.

I suspect such trailers would be available for hire somewhere in Britain. On a large site such trailers would not be out of place.

Cheers, Rowan