In an unitary constitution all legal sovereignty is contained in a single place - for example central government. Britain’s constitution is unitary. The UK has no states, so whilst Central government can choose to give out powers to devolved assemblies and local councils it can take those powers back at any time as they are not guaranteed by the constitution. This has happened with Northern Ireland when, for instance, the Stormont government was suspended as the Nationalist and Unionist politicians couldn’t agree on power sharing. This is contrasted with federal systems like the USA, Germany, power is divided between a central (federal) government and various states (shared sovereignty).