Helpful video reference. SoftWash Guy's video "How to Chemically Clean Render (extreme cleaning)......Soft Wash" shows the chemical soft-wash process on render.
Why Dover render is a different problem
Salt carried on sea air settles on exterior surfaces and, combined with the moisture that comes with a coastal climate, speeds up the growth of algae and general grime on render. Properties nearer the seafront and the port typically need cleaning more often than the same house would a few miles inland.
Why high pressure is the wrong tool
Render is a thin decorative coating over blockwork or insulation. Direct high pressure can blast through the surface finish, crack it, or force water behind the render where it doesn't belong. Once render is damaged this way, the fix is a re-render, not a clean.
What soft washing does instead
Soft washing uses low pressure combined with a proper cleaning solution that breaks down algae, lichen and grime chemically, then rinses it away gently. The dirt is dissolved rather than blasted off, so the render's surface finish stays intact.
What to check before any clean
Walk the wall first for loose render, cracked paint, or hollow-sounding patches, and check for hairline cracks that might let water in. A discreet test patch first is standard practice before doing the whole wall.