Yeah if you're behind someone looking to overtake you might be giving them more space in the braking zones by coasting, and on the straights you're have less drag which will reduce fuel consumption. Loads of variables like Ozwheels says, we should look into it more before coming to a solution, and that's just small stuff. On a smaller track like Brands Hatch I assume that as it's a slower track fuel consumption is going to be much lower than on a track like Road America which has lots of high speed sections.
I'm happy to do help out with any testing that needs to be done.