What I think is missing from this market at the moment is an honest home service, not the usual crap that people peddle.
These mobile valeters these days can be just as bad as taking your car through the £2.50 car wash with some of the junk they use.
What I would advise is rather than setting up the traditional mobile valeter with van, lots of chemicals etc etc, why not see if you can pitch your local area and clean other people's car in the same way you'd do your own?
So, using their water, their power, you'd then only need a decent Karcher washer or similar and then some decent detailing stuff, nice cheap way to start and then people would see you're doing a proper job and pass the word on. Not quite as mobile as you'll have to do this at people's homes but not a bad place to start without having to lay out lots of cash?
Just a thought anyway