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 Canterbury jobs
Canterbury's inland, but the River Stour runs right through the city centre, and a lot of the older streets around the historic core drain toward it rather than the coast. It's an easy assumption to make that being away from the sea means drainage doesn't matter as much here, but a surface water drain fifty yards from the Stour reaches the river just as fast as one on a seafront road reaches the Channel.
Sources: Pure Seal Services, "Pressure Washing Environmental Compliance Guide (UK)"; GOV.UK, "Discharges to surface water and groundwater: environmental permits".