The British Taxpayers Alliance got carried away a bit when it said taxes as a share of British GDP have not varied much over the last 50 years or so. Margaret Thatcher would be turning in her grave.
A stable tax take is more the case in the USA. Federal tax receipts stay within the range of 18-20% of U.S. GDP as shown in the charts below and above.
There were large cuts in the top tax rates in the USA without any fall in tax revenues as a percentage of GDP because of base broadening.
Margaret Thatcher really did make a dent in taxes as a share of GDP in the 1980s. They fell by 5% of GDP but then went back up again in the 1990s as is shown in the Centre for Policy Studies chart below.
That 5% drop was a big variation as a share of GDP which is also shown in the Taxpayers Alliance chart if you look closely at the 1980s. That sharp drop in taxes as a share of British GDP is clearer in the Centre for Policy Studies chart because it magnifies the data.
There are also big changes in the British tax mix in the 1970s and 1980s. The large rise in tax in personal income in the 1970s as a percentage of GDP, also shown in both British charts above as well is the one below, coincided with the rise of the British disease and British economy becoming widely known as the sick man of Europe.
Source: OECD Stat.
The large decline in taxation in personal income under Thatchernomics was followed by an economic boom. The UK grew at above the trend annual real GDP growth to 1.9% for most of the period from the early 1980s to 2007 as shown in the detrended data in the chart below.
Source: Computed from OECD Stat Extract and The Conference Board. 2015. The Conference Board Total Economy Database™, May 2015,http://www.conference-board.org/data/economydatabase/.
In the above chart, a flat line is growth at the same rate as the USA for the 20th century, which was 1.9% for GDP per working age person on a purchasing power parity basis. The USA’s trend growth rate in the 20th century is taken as the trend rate of growth of the global technological frontier.
A falling line in the above chart is growth in real GDP per working age person, PPP at less than the trend rate of 1.9% per annum while a rising line is real growth in GDP per working age person in excess of the trend rate.