British nationals can sign up for a Canadian ETA before the flight to reach Canada by air.
For business or tourism purposes, UK visa holders with a confirmed ETA may stay in Canada for as long as six months. Although the exact duration permitted can vary, the maximum 180-days stay is given to most British citizens. Similarly, a Canadian ETA is necessary for a UK public to fly through a Canadian air terminal when coming up and leaving by air. British citizens who wish to stay in Canada for more than six months should receive a regular visitor visa depending on their travel intent.