The Office for National Statistics (ONS) has released their Annual Survey of Hours and Earnings (ASHE). It relates to employees but gives an idea of what a self employed person needs to be aiming for. (And that's where tariffs come into the frame).
The median national average wage = £447.10 (that's the figure below which 50% of full time employees earn per week)
The mean national average wage = £537.30 (that's the arithmetic average of all the full time employees)
The mean figure is the relevant one from the point of view of the taxi trade, because this is the figure that relates to all employees and not just the lower 50% and is thus more representative of the average taxi passenger.
[url]http://www.statistics.gov.uk/StatBase/Product.asp?vlnk=14630[url][/url]