while i agree it would have made more sense to extend it to Canada, i don't know how the delay in the UK would be a primary issue. if we assume they were tracking UK sales before the variants were issued in the US, then why wouldn't they get variant issue sales reporting in the same time frame they would have normally gotten for their regular priced issues?
i'm sure it didn't take six months for them to figure out they were going to go to an increase in November 1977 for books sold in the U.S. they probably had all the data they needed by the time the September issues hit the stands...
i must be missing something.
i think a more likely reason is something like the UK not providing a wide enough geographic distribution to hide the two prices, combined with Marvel perhaps not sending enough books over to the UK to run both prices at the same time