Actual fight tickets are not required, just indicative plans and reasons to return.
Likewise when entering the UK Border, whilst some Border Force Officers may well ask to see one, because it’s an easy route, it isn’t a requirement.
That said, they can ask the passenger to demonstrate that they have the means to leave the UK at the conclusion of their visit.
The UKBA are more than aware that possession of a return ticket, doesn’t mean they intend to use it.