Airmail rates are given in the airmail leaflets (
https://www.gbps.org.uk/airmail-leaflets), but for a more fine-grained view the best book is Michael Furfie's
British Civilian Postage Rates of the 20th Century. Only about £6 and one of the essential books -- I think he's still selling them,
mrfu54@yahoo.co.uk.
In this case, looking at the book the airmail surcharge to Canada was 7d from 31st October 1928 to 3rd November 1929 after which it became 4d, so added to the regular 1½d postage that does indeed give rates of 8½d and 5½d, so that checks out.
(Pre-1947 airmail rates are definitely on the "to-do" list for the website, but unfortunately that list is a long one! I have made a start with a draft version of a listing for Europe which will hopefully appear sometime soon, though, and others in due course.)