As British visitors to Canada arriving by air, must, as of November 2016, have an Electronic Travel Authority to travel to Canada. Those planning trips for 2017 are advised to apply in advance of booking flights. The ETA costs $7.00 and is valid for five year. Purchase is done online through www.cic.gc.ca/english/visit/eta.asp.
Please note : Any site charging more than $7.00 is not the official government site.