Ofcom has a set of General Conditions of Entitlement (GCs) which are published on the Ofcom Website. These GCs establish a comprehensive set of regulations governing electronic communications networks and services. They particularly draw your attention to Part C6 of the GCs, which outlines requirements for Calling Line Identification (CLI) facilities. To support this industry compliance, Ofcom has published detailed CLI Guidance.
By understanding these guidelines, CPs can mitigate the risk of enforcement actions and substantial financial penalties. This is especially important for CPs who sub-allocate telephone numbers (see their - good practice guide) to prevent potential misuse by scammers.