Eh, Google Maps is capable of doing this all by itself. Go to Britain. Lay in a course from somewhere in northwest Birmingham, down to Maidstone (southeast of London). If you've done it right, there should be routes offered via the M40 and the M1, amongst others; the former ultimately going around the southwest of the capital, and the latter around the northeast.
Make sure you're zoomed to see the whole route. Select one of those alternatives and mouseover the other, so that both lines are displayed.
Voila. Dogcock. Trolled by motorway route engineers over a period of 50ish years. Epic.