As speed increases, aerodynamic forces become much more significant in the amount of energy required to maintain that speed. Keeping this in mind, consider the following:
For argument sake, we'll use the aero numbers I came across for the the Boss 302 Mustang, with a frontal area of 24ft^2, and around a .36 drag coefficient. From best data I have been able to find for a Caterham we'll call the Locost a frontal area of 16ft^2, and around a .65 drag coefficient.
If anybody has more accurate numbers or formulas, please feel free to correct me.
The numbers I've seen for a Locost is that it has a Cd of 1. But cheer up, it's the same as the typical F1 car.
If you want good fuel mileage you don't want a high powered car. You aren't going to get high performance either. And the shape of a Locost is at the wrong end of the scale of what you need.
You want to get 40++ mpg the easy way?
My 1988 Honda CRX HF with a Cd of .29 and only 60 hp gets 42 to 48 mpg in town and 51+ mpg on the highway at reasonable speeds! The 1.5L engine was designed/tuned for max efficiency not hp. So you have to choose your engine accordingly.
One of the secrets is to have max torque (best efficiency) at the rpm your cruising speed will be. And don't expect that speed to be in the 75 to 80 mph range either with less than optimum streamlining. The CRX HF has max torque at 2000 rpm which equates to 64 mph.
Another secret if you're using a throttled engine (not a Diesel) is to have a vacuum gauge right in front of you and keep the vacuum as high as you can all the time.