Based on two posts discussing how external links work in Canvas:
I'd like to suggest the following, I'll pitch this in the form of a User Story:
"As a User I would like the behaviour of offsite links to be correctly determined so they open in a new tab/window. The offsite link should display an icon on the page, an example is given below where the google.co.uk link shows as an external link."
However due to the way Canvas works, offsite links to other educational establishments are not treated the same and open in the same window. Canvas treats .ac.uk domains as if they are a cross site link and opens them in the same window."