We analysed the use of language in UK parliament debates from December 1935 to March 2010. The terms of recent Prime Ministers are highlighted at the bottom of each graph for reference. It's also worth keeping in mind that Alistair Campbell became Director of Communications for the Labour Government in the year 2000.
We used the parliamentary debates raw data provided by the excellent They Work For You website. Common words (the, at, honorable, minister, in, of, order, debate, sir, and so on) and infrequently used words were removed, with the remaining words grouped into a database by year. Note that the data for the years 1935 and 2010 is incomplete — we only used the data from the 26th November 1935 to 31st March 2010 — and so the statistics for the first and final years may not be reliable.
Each year differs in the number of debates, and hence volume of data. Therefore, rather than analysing the absolute count of usage for each word, we instead compared the count of each word against the total number words recorded in our database for the year — resulting in a percentage, which is more reliably comparable across years.