As a European citizen planing to go to Peru from the UK, I need a round trip ticket even if I don't plan to return at all. With just a one way ticket, I may be required to book a return flight at the airport since I don't hold any long term Visa for living in Peru.
What about a non-European citizen holding a 5 years spouse British Visa travelling from Peru to the UK? In this case, can airlines require a return ticket as well even when she doesn't plan to return?
In my experience, airlines typically do not require a return flight for any long-term visa (where "long term" is a year or more). I recently found it impossible to book (on Air New Zealand) a round-trip flight for somebody with a return date more than one year after departure. The airline will expect you to purchase a separate return flight.
What matters most is the UK custom. If you have what you need to comply with what they require, then there is no reason why the airline will not let you on board.
What you typically need is a proof of leaving the country. It does not mean that you go back where you came from. What it means is that you have booked a plane, train, boat or bus ticket to get out before the legal stay limit.
For instance, I went to travel to Indonesia for an unknown length of time. I still bought a ferry ticket out of the country (to apply for a visa & to get in without problem at custom), which I never used. This is because I extended my visa to stay longer.
I suppose you can do just the same in almost any country.
Another technic is to simply find a booking website that allows you free cancellation. So you book two plane tickets, and then cancel the return. I met a few people who did that. Not sure how risky it is though, one could lose quite some money if the cancellation is declined for some reason. I heard Orbitz does good free cancellation. Again, watch out for the terms and conditions for canceling!
