CBEST is not a blanket statutory obligation in the way that, for example, certain reporting requirements are, but participation is strongly expected - effectively supervisory-driven - for banks, insurers, and financial market infrastructures (FMIs) that the Bank of England, PRA, or FCA consider important to the stability of the UK financial system. Firms identified for CBEST are typically approached directly by their supervisor.
Describing it as either fully mandatory for all firms (C) or purely voluntary with no regulatory interest (A) misrepresents this supervisory-driven, risk-based approach, and eligibility is based on systemic importance, not physical location (B).