Whilst a mini is out of it for mega miles and a jag out of it for economy why dont you approach this slightly differently as economy cars will be fetching quite a premium at the mo I would have thought.
Work out how much money youve got to spend then have a look at how much a car you like would cost to run for the mileage youre planning. People get far too hung up on mpg. My missus car returns phenomenal mpg but cost her £18 000 !!!
You may be able to buy a big or boring motor dead cheap although it'll cost you a tad more in fuel but at the end of the day has cost you the same per mile.
Take into account everything like cost of exhausts , tyres and insurance.
Ive done that a few times and had some pretty good cars :thumb2
Good example of getting it wrong was me with the jeep, tyres turned out to be pricey (i didnt check thoroughly), got it right with the Seat i had, dead cheap to insure not wildly economical fuel wise but fast as hell to drive (golf 16v engine) and dead cheap to buy and service :thumb2