Terrorism Act 2000 (which was partially ruled unlawful by the House of Lords for what its worth) allows for theoretically infinite extensions to the detention period with only a police superintendent needed to sign off on it.
as for imigration 'Just over one fifth of immigration detainees are held for at least two months' :
as for imigration 'Just over one fifth of immigration detainees are held for at least two months' :
https://migrationobservatory.ox.ac.uk/resources/briefings/im...