Mean and Median Income (Personal incomes by tax year) from the Survey of Personal Incomes by HMRC. These are estimates based on a survey and should be treated with caution. They are based on the Survey of Personal Incomes (SPI) an annual sample survey of HMRC records for individuals who could be liable to UK Income Tax.
Further data on self-employment income, employment income, pension income and total tax are available from the HMRC website.
Here is a GLA Intelligence Update analysing this data in 2007/08: