Thank you for the reply and apologies for taking so long to get back.
Running xdg-open
http://some/url in the terminal opens a browser session, as expected. So does opening urls from other applications e.g. Sublime Text; the problem appears to be restricted to Okular. Is there anything else Okular might be using other than xdg-open? Or perhaps that urls embedded in a pdf lead to different behaviour (via xdg-open) than those found in plain text?