As of March 15th 2016, there are new requirements for those visiting Canada meaning that UK citizens (and others) require an ETA (electronic travel authorisiation) to enter the country.
I wouldn't want to mislead anyone so I suggest you look on the Canadian government website
http://www.cic.gc.ca/english/visit/eta.asp for the details and a link to apply.
The ETA costs $7cdn for 5 years.
