Welcome to Flyertalk LucyDan.

As airoli says, you'll unlikely be asked and the ticket back to the UK will sufficient for CBSA (Canada Border Services Agency). What CBSA wants to know is that you aren't going to squat in the country without a proper visa. If you are moving on, however it is, as long as it is plausible they won't care.

I would expect that just saying "we are staying 3 days in Vancouver and then driving with a friend down to Seattle," would be enough of an answer. It is unlikely they will question more, but showing the ticket onwards should answer any of their concerns.