Fill your boots, I'd suggest the cheapest way is not to drive it at all ... 😉
Its a public forum, unfortunately you cant control what responses you get, but the short answer is the cheapest way to charge is overnight on an EV tariff. I'd assume that if you set the sat nav for your destination, the car will decide the best way to deploy the battery power during that journey. Wont it?
Anyway, enjoy the car.