Can I enter Canada on a one way ticket with a British Passport?
You MIGHT get into Canada, depends on the agent that you talk to, and what questions he asks. Especially your answer to "how long will you be in Canada" Getting into the US, might be a bit harder, without that ticket. Advice, buy a greyhound ticket online, Canada to US, this will please the Canadians CBSA officer, cant help with the US border guard, unless you buy your San Diego - South America ticket, before crossing.