Immigration officer placed a standard Leave to Enter stamp when I have Indefinite Leave to Remain. What do I do? [closed]

I'd contact the Duty CIO or HM Inspector at Gatwick and explain the error to them. You will be called back to Gatwick and the endorsement will be amended. The Home Office Referred Casework Unit at Lunar House only deals with entry clearance/ visa amendments but the incident happened at the Port of Entry and therefore you need to contact Border Force at Gatwick. Next time make sure the IO looks at your expired passport and endorses the free standing date stamp with VIPP.