Well, it'll do a reasonable job of removing your previous wax!
Washup does contain very aggressive detergents, to get grease off pans. In general terms, it is stronger than you need for a car wash, so why use it? Car shampoo isn't that expensive.
and don't get it on your windscreen!!
It leaves a film that will cause 'starring' from headlights, streetlights and the sun, making it difficult to see.