Just going back to the source of this discussion - for 7000 miles a year, the differences between these cars fuel economy is going to be minimal. As you've already said, you're getting way below average mpg on your current car because the majority of your driving is cold / stop start stuff. So, comparing a car with a theoretical mpg of 55 in ideal conditions, with your real world 30 mpg will be misleading.
So, I guess it really comes down to - do you just want to buy a new car? In which case - considering you don't do all that much mileage - just get a car you like and be damned with the economy differences.
If you really want to save money, try out a hybrid, and don't need 4 seats - how about a
Mk1 Honda Insight. I say save money, because you'll be able to double your fuel economy even on your journey AND the car will likely hold it's value indefinitely. Most cars you've been quoted before this will cost you more in depreciation than fuel.