Qualified agreement.
Do your calculations take the difference in road tax into account? Also, anyone doing a significant mileage on the continent is going to save much more on fuel costs with an MJ. There are also the questions of reduced emissions from the MJ, and the difference in performance, which are more subjective but important to some people.
We don't have road tax in Guernsey
We did have it but now we pay more petrol tax instead ( our current unleaded price now is about 93p per litre on average )
I agree in the UK this needs to be taken into consideration.