I have a cover sent from London to Vancouver in 1929 with 8.5d postage and an Air Mail sticker. I know that there was no Trans-Atlantic Air mail at the time so this applies to the Canadian portion of the trip.
I have seen another cover sent in 1930 from Southend-on-Sea to Clinton, Ontario with an Air mail sticker and 5.5d postage.
There is no mention of these rates in this period on the GBPS Postal rates site so I am sure it is complicated. Does anybody know where I can find information on Airmail rates from GB to Canada prior to the trans-Atlantic service?
Many thanks for any information!
Dave Russum

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.)

That's perfect - many thanks for bringing the Air mail Leaflets to my attention, I hadn't seen them before. Now I understand how complicated early Air Mail rates can be and why there is not a simple way to access a summary!
Your work in this area is much appreciated.
Dave Russum