To supplement Post 43 here is some information from Robin Higham's history of the British Overseas Airlines, which was my main source for Post 43.
The first figure is the Net Air Estimates for the financial year and the second is Vote 8 - Civil Aviation. The third figure is the amount I propose to spend ITTL. Note that the increase in spending is not quite what I proposed in Post 43. It is quadrupled from 1922-23 to 1936-37 and doubled in the other years.
I'm not sure if Vote 8 is the gross or net figure. Furthermore Higham's figures might not include supplementary estimates as I have not crosschecked them against my other source, which is the articles on the Air Estimates from Flight Magazine which can be downloaded from the Flight Archive website.
The spending from 1924 to 1931 may or may not include the cost of the Imperial Airship Scheme, which might have been in the vote for research and development.
The overall increase from 1919-20 to 1938-39 is nearly £30 million, but the Government should get some of that back through increased taxes collected from the aviation industry and indirectly from the expansion of commerce that the speeding up of the mails was intended to create.
1919-20 - £ 54,030,850 - nil - £2,000,000
1920-21 - £ 21,056,930 - £ 894,540 - £ 1,789,080
1921-22 - £ 18,411,000 - £ 880,000 - £ 1,760,000
1922-23 - £ 10,895,000 - £ 364,000 - £ 1,456,000
1923-24 - £ 12,011,000 - £ 287,000 - £ 1,148,000
1924-25 - £ 14,511,000 - £ 355,000 - £ 1,420,000
1925-26 - £ 15,513,000 - £ 357,000 - £ 1,428,000
1926-27 - £ 16,000,000 - £ 462,000 - £ 1,848,000
1927-28 - £ 15,500,000 - £ 464,000 - £ 1,856,000
1928-29 - £ 16,250,000 - £ 415,000 - £ 1,660,000
1929-30 - £ 16,200,000 - £ 450,000 - £ 1,800,000
1930-31 - £ 17,850,000 - £ 500,000 - £ 2,000,000
1931-32 - £ 18,100,000 - £ 470,000 - £ 1,880,000
1932-33 - £ 17,400,000 - £ 473,000 - £ 1,892,000
1933-34 - £ 17,426,000 - £ 490,000 - £ 1,960,000
1934-35 - £ 17,561,000 - £ 513,000 - £ 2,052,000
1935-36 - £ 20,650,000 - £ 595,000 - £ 2,380,000
1936-37 - £ 39,000,000 - £ 760,000 - £ 3,040,000
1937-38 - £ 56,500,000 - £ 2,315,000 - £ 4,630,000
1938-39 - £ 73,500,000 - £ 2,925,000 - £ 5,850,000
1939-40 - £ 66,561,000 - £ 4,787,000 - £ 9,574,000