Postage rates 1930s-1940s UK
Postage costs were by weight rather than shape, volume or speed of delivery. There was no First or Second Class. As special recognition of the Empire (and possibly also the emerging Commonwealth), postage by Sea Mail anywhere there cost the same as within the UK.
It is worth tap/clicking to magnify the postal rates image, not only to see the prices, but also to note how punctuation has changed over the years. Whereas we might put a full stop only at the end of a complete sentence, full stops were placed at the end of phrases and headings. Also note the names of countries that no longer exist or have been renamed, and of course notice the old, pre-decimal money.
The booklet was undated, but it's date can be estimated: I gives a list of presidents of the USA, the latest of whom was President Roosevelt. Since he was in office between 1933 and 1945, that dates the booklet to between these dates. Also as it was almost certainly bought by my grandfather who died in 1942, the booklet must have been produced between 1933 and 1942.
I have no idea if or how postage rates changed during these years, but offer the booklet for your interest. You may like to look at the postage stamps page to gain an estimate. The economy mail page considers air mail postage.
How's this for postage inflation!?
On January 1 1937 a postage stamp to send a letter was equivalent to one penny in today's money, [tuppence ha'penny, i.e. two pennies and a half penny in old money]. On January 1 2020 it is 85 pence First Class.
Based on a letter in The Daily Telegraph 08/12/20
In 1937 of course, the vast majority of communication was by letter whereas now relatively few letters are sent. Consequently the Post Office has to increase its prices to maintain its income.
So, as always, it is not straightforward to calculate inflation on the basis of price increases.