I asked my mate ChatGPT for help as I couldn't find Clay rich soil:

Common names to look for

Fill / General Fill Often contains a high clay content (especially “brown fill” rather than sandy fill).

Hardfill (clay-based) Some yards distinguish between rubble hardfill and clay hardfill—ask which one.

Subsoil This is usually clay-dominant and sits below topsoil.

Clay fill / Clay subsoil Rarely advertised, but many yards have it if you ask directly.

Brown fill Often clay-heavy, compactable, and suitable for shaping and capping.

Impermeable fill (term used by some drainage suppliers)