Whenever you flush the loo, take a shower or use the dishwasher, the used water disappears round the U-bend, never to be seen again.
It leaves your property via a private drain which serves only you and is your responsibility to look after. At the point your drain connects into a pipe which serves more than one property, or the drain runs beyond your property boundary, the local water authority become responsible for it.
The illustration shows you how this may work.
Surface water drainage occurs when rainwater falls on a property and drains away.
Most rainwater falling on properties drains into public sewers owned by the ten water and sewerage companies in England and Wales. The water companies are responsible for removing and processing this rainwater. The companies collect around £1 billion each year to cover the costs of this service. If rainwater drains from your property into a public sewer, you will be charged for surface water drainage through your sewage bill. If rainwater does not drain from your property into a public sewer, because you have a soakaway or similar, you may be entitled to a surface water drainage rebate.
Water that drains from roads and footpaths flows into public drainage systems. This is known as highway drainage. Water companies recover the costs of providing highway drainage from their customers. Highway drainage benefits everyone that uses the road system
Above info from OFWAT website and may change from time to time