An eta is not a visa. UK citizens do not require a visa to enter Canada, and they can do so without an eta by other means than air. (i.e. By road or sea).
When the pax checks in the DCS will give a board / do not board response. Perhaps this can be over-ruled. The 9 Nov 'grace period' suggests that it can be.
My British wife entered Canada in Sep with an eta. It appeared to be a much quicker process, with fewer questions, but that could have been an illusion.