What is the equivalent of an Australian or New Zealand honours thesis in British universities?
in England, honours is granted after three years without a mandatory thesis or essay component. Scotland is more similar to Australia but not identical. 

Even in Australia, the requirement for an honours thesis varies. I don't see why you can't be explicit. Explain that honours is an optional fourth year similar to a one-year English masters and detail what fraction of the year was spent on the thesis. If your students' theses were publishable, get them published and say they led to publications.