Any possible problems using One-Way flight into UK?

[TravelNiki]

I booked my one way through Itravel2000 and called back 4 times to talk to different agents about this question. They said that you definately don't need a visa to get into the country, and should have no trouble getting in on a one way ticket.
My sister said that it is often cheeper to buy a ticket returning to Canada from London when you're there.
My question: If you are on a 5 yr ancestry visa with the intention to stay, how can they expect you to purchase a return ticket?
Does anyone have a reliable source to contact? Not that I don't trust ITravel. . . .

[retrobolted]

Be aware that you might not be allowed to board the flight in Canada. I booked a one way ticket to Germany a few years ago, with the thought that I would buy the return ticket when I got there, as I planned to stay for the summer and wanted flexibility. I head off to Pearson and the airline would not let me board the plane! I had to purchase the return portion on the spot, at whatever ridiculous price was available.