CINCFLEET command headquarters are in Portsmouth ; operational headquarters are at Northwood , Middlesex . CINCFLEET command is exercised through the Navy Command Headquarters , based at in Portsmouth . An Operational Headquarters , the Northwood Headquarters , at Northwood , London , is co-located with the Permanent Joint Headquarters of the United Kingdom 's armed forces , and a NATO Regional Command , Allied Maritime Component Command Northwood ( AMCCN ) .