Utilize your financial knowledge, give your answer or opinion to the input question or subject . Answer format is not limited.

How were self employed folk taxed in the U.K. before 1997
This link:  http://www.ifs.org.uk/fs/articles/ewgm_feb93.pdf (from 1996, describing the proposals for the change) seems to answer the question in its description of "the current system" - they had to file business accounts and it was calculated by the Inland Revenue from that.