Think that is an issue from many years ago, several reasons...
1/ ON-line CHeckin means a lot of passengs are already hold an onward boarding card when they arrive into Lhr.
2/MOst passengers thru check baggage and with today's security levels, they can-t just switch flights when they arrive in LOndon.
3/E TKts have to be endorsed over in order to change carriers
4/many Promotional fares are non changeable and if another carrier did poach a ticket they would be carrying page for nothing and the original carrier would keep the money without having to carry the psgr.
5/The era you refer applied many to passengers originating in LOndon and turning up ,not checked in and without a boarding card and they needed to be on flexible tickets.
6/even if a way around all these issues was found by an over zealous staff member ,company procedures outlaw such practices and would involve serious disciplinary measures I.e. probably termination.