Hello Suzie:
I can't give you a definitive answer, but my educated guess is "I doubt that you need an onward ticket".
I say this because if you are a British Citizen, you don't need a visa to visit Canada. What you do need is an 'Electronic Travel Authorization' (ETA), which is not quite the same as a visa. I'm not exactly sure what an Electronic Travel Authorization is, but I suspect it is a method by which the Government of Canada can find out a little bit more about a person prior to arrival.
Here are two links to Government of Canada websites that might help you:
Find Out if you need a Visa or ETA (select 'British Subject' - the country 'United Kingdom' is not available as a choice).
ETA Requirements for Canada
Note that you only require the ETA if you are entering Canada by air - if you enter by land, you don't need it.
Michael
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