Gross oversimplification. Octane required higher in some UK models (less petrol/gasoline per bbl. crude). In any case, gasoline/petrol prices much higher in UK.

For U. S. models, see government figures, http://www.fueleconomy.gov (remember, these are U. S. gallons, smaller than Imperial gallon of UK and Canada)

For Canada, see Natural Resources Canada, Fuel Consumption Guide,
http://oee.nrcan.gc.ca/sites/oee.nrcan.gc/ca/files/pdf/transportation/fuel-consumption-guide-2011.pdf

There are several reasons why figures for U. S. and Canada differ; you might explore that as well. Different calculation factors used, greenhouse gas emissions allowable different, etc. (Also don't forget to calculate difference due to U. S. vs. Imperial gallons).

In any case, compare price- higher in UK. Equivalent cost U.S. $4, equivalent in UK about $7 (Canada higher than U.S. because of more taxes).