I think you'd be safe from prosecution flashing a fake warrant card. I understand to actually get a successful prosecution for impersonating a police officer they would have to catch you letting the taxi driver go, whilst arresting yourself for having the temerity to be hit by a motor car, followed by teararseing down the road at 100mph with blues and twos to get a takeaway, before beating an innocent bystander to his death.
I think you'd be safe from prosecution flashing a fake warrant card. I understand to actually get a successful prosecution for impersonating a police officer they would have to catch you letting the taxi driver go, whilst arresting yourself for having the temerity to be hit by a motor car, followed by teararseing down the road at 100mph with blues and twos to get a takeaway, before beating an innocent bystander to his death.