Lockdown has no provenance as a legal definition in British English. It’s likely origin in the UK is in connection with making machinery safe (19th century). As a term for restricting freedom it was first used in reference to containing prison disorder in the USA in the 1970s.
Boris (or rather his speech writers and advisers) have wisely opted to describe in simple terms what the restrictions entail rather than giving them a sensationalist title that is open to misinterpretation.
https://www.etymonline.com/word/lockdown