Never seen a fight!
/Thread drift.
They check documents before you board so that if you declare you 'do not have them' for any reason - they can prove you wrong.
A relative of mine used to work the LHR Immigration 'beat' for a number of years and they said that aircraft toilets were the favourite place to find the 'lost' documents.
It's also why with arrivals into the UK from certain destinations, there are plain clothes Immigration staff on the Jetway. If they can stop the person BEFORE they make any claim, it's a faster turn around.
Once, on arrival at LHR, my relative was one of two staff and their colleague was astounded to see a passenger greeted so warmly!