It is based on statistics taken from the Digest of United Kingdom Energy Statistics 2008, Table 1.1 – Energy Balance 2007. The flow chart is a simplification of these figures, illustrating the flow of primary fuels from the point at which they become available from home production or imports (on the left) to their eventual final uses (on the right). They are shown in their original state and after being converted into different kinds of energy by the secondary fuel producers. The flows are measured in million tonnes of oil equivalent, with the widths of the bands approximately proportional to the size of the flow they represent.
More detailed flow charts are available by fuel on the BERR website.
I previously posted WRI’s world greenhouse gas emissions flowchart.