Tell them to wash the under carriage with high pressure water ( no oil / diesel spray ). Once done, take the car home, spray the degreaser in the wheel arches and get to work.
I do have a car stand where I can access the car from below with ease at home so that's a plus. Also, we have a water boring machine due to which insane water pressures isn't an issue. I can use a valve to adjust the water pressure. Honestly, I've even stopped going to service stations walla.