Every heavy vehicle has legal mass limits: per axle group and gross. General Mass Limits (GML) apply by default; Concessional Mass Limits (CML) and Higher Mass Limits (HML) allow more payload in exchange for NHVAS mass accreditation and, for HML on many routes, telematics-based monitoring.
Overloading is a Chain of Responsibility offence for everyone in the chain, not just the driver. The heavy vehicle rules guide explains the schemes in plain words.
General information, not tax or legal advice.