Canada has made eTA (Electronic Travel Authorization) a mandate from this month onwards. This is similar to ESTA (Electronic System for Travel Authorization) in the USA.
Though eTA was introduced in March, British passport holders were given some exemptions. On 29th September this exemptions will get expired and all the foreign nationals who are travelling to Canada after this date will have to have an eTA or a valid permit.
eTA can be obtained by CIC website at a cost of CAD $7. Other websites also provide the same service with some extra charges. An eTA will be valid for 6 months to a period of five years or until the passport expires.
Canadian government suggests applying for the permit before booking the ticket to Canada, as the process can take up to 2-3 weeks too, even though the online system often grants it within minutes.
Even if someone is travelling via Canada using a connecting flight, they need to have an eTA to be at the airport while transit. The only exceptions are given to Brits travelling by land or sea. This will be applicable to all the citizens from visa-exempt countries as well.