Detailed answer to this question from the Vegetarian Resource Group. Asserts many surveys in the past have been flawed since millions of people think they're vegetarian, but are mistaken about what the word means.

Online research panel publishing a large number of statistics concerning the needs and wants of 21st century vegans.

Large number of statistics from surveys from 1945 to the present day, covering the number of people in the UK who called themselves vegetarian, or avoided certain animal products, and attitudes towards vegetarianism.