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!