I understand that. My Jeep does 21-25mpg but on the plus side, it does very little annual mileage - maybe 2000. My wife has an old 04 Saab 2.2 diesel which returns 50-55mpg. Big difference..
Have a look at British Car Auctions website: http://british-car-auctions.co.uk/ and sign up (it's free). There's probably an auction centre in your area.
There'll be 2 auctions a week. 200+ cars per auction. You can see the cars being auctioned on their website, but no estimated prices. You have to be there on the day.
Depending on the price you pay for a car there's also a buyer's premium. You'll be surprised at the prices.
Just out of interest, here's a link to a car being sold by a dealer straight opposite my house. He's now asking £2495 for it after having dropped the price by £400 ..