Depends where you want to work, but in general you'd gain admission to a grande ecole by having a bachelor's degree already so you could just do your bachelors at Imperial then spend the rest of your time at the grande ecole, that is a very expensive option however.
Edit: ignore that, I think masters at grandes Ecole's may be limited to French nationals, in which case Imperial would be your best option or go to a normal French uni like Central which would be much cheaper.