The rule in one sentence
Wastewater from a pressure wash, whether it's just dirt or has detergent in it, must never be allowed to run into a surface water drain. Causing polluting matter to enter controlled waters, rivers, streams or the sea, is an offence under the Water Resources Act 1991, and the Environmental Protection Act 1990 puts a duty of care on anyone doing the work to prevent it.
Surface water drain vs foul sewer
Most UK streets have two separate underground systems. The foul sewer carries wastewater to a treatment works before it reaches a river. The surface water drain, usually the gully at the kerbside, exists purely to carry rainwater away, and it goes straight to a river, stream or the sea untreated. A pressure wash that runs into the wrong one is exactly the pollution the law is aimed at, regardless of intent.
How to tell which one you've got
There's no universal marker, so the safe approach on any job is to assume a driveway gully is a surface water drain unless it's confirmed otherwise, and act accordingly: contain runoff, use a biodegradable detergent, and direct water away from the gully rather than into it wherever the slope allows.
What this means for Folkestone jobs
Folkestone's harbour and The Leas clifftop sit right above the sea, and the town also carries constant HGV and coach traffic film from the Channel Tunnel terminal, which means driveway runoff here often has more than just dirt in it. On jobs near the harbour or up on The Leas we check drainage first and use a biodegradable detergent as standard, given how close most of the town is to open water.
Sources: Pure Seal Services, "Pressure Washing Environmental Compliance Guide (UK)"; GOV.UK, "Discharges to surface water and groundwater: environmental permits".