Keep the Civic.
No? Well if you've got a budget of £4k you can get a very nice Mk3 Mondeo TDCi, which should very easily to 50mpg+ on a motorway/A-road commute.
I take it the pay rise you'll be getting in this new job is more than enough to pay for the extra fuel and 5+ extra hours a week sat in a car? I'd have to be looking at serious money to do a commute like that
TBH....and if I did get the offer, I'd be saving the extra up to buy a house much nearer to where I worked a couple of years down the line.