Firstly, I never let anyone wash my car..I just don't trust them.
However, I have a product called Astonish Pre-wash car treatment which you use on the really dirty places before setting-to. They seem to have discontinued it, though. And Astonish do a range of inexpensive car-care products, like this...
http://www.astonishcleaners.com/product-car-care-6.html
...all readily available. See list of stockists.
If you don't mind paying more, there is a fantastic range of products on this site...I'm sure you'll find what you are looking for:
http://www.polishedbliss.co.uk/