So my wife bought a VW Polo earlier on this year from the bluemotion range. We wanted something really quite fuel efficent I have have to say we are really quite pleased with how it lives up to the hype.
Based on the data provided by the onboard computer we average over 70 mpg, on longer motor way journeys it is not unheard of to average over 80 mpg. (Note these are UK not US MPG. Use a converter such as this one).
Remember this is the model with air-con and leather trims so it is not some eco-hay waggon. The variable speed turbo charger also means that is performs much better than you would expect from a diesel. Indeed I enjoy driving it more than my other sportier car in many ways.
And yes you could quite happily drive from London to Edinburgh on one tank of fuel, and avoid the congestion charge in the future due to its very low CO2 emissions.