I need to buy a motorway cruiser to travel around 40k miles a year as economically and comfortably as possible.

I'm thinking of travelling at mainly in the outside lane, and spending around £10k to minimise depreciation (given the car will be worth a fraction of that after the 3 years I was thinking of keeping the car). Is this a reasonable premise? What car and engine type will best fit this need to compromise performance, cost and fuel economy without leaving me a wreck at the end of each journey, and where best to source it?