It's fine, because you are not selling a product, just a service. You need to ask permission at a Tesco's, but some carparks which are public property might be ok just to wing it. You would also need to offer proof of the proceeds going to charity. Most charity work is 100% proceeds to the charity, and you volunteer your time, minus costs like uniforms, and cleaning materials.
Best to be honest and just say you are trying to make a little money part-time, and if you make 1000 then you will donate 100.
Go and get a T-shirt printed and a Hat with matching colours so you have a little bit of a "uniform" to wear. 0 or 30 pounds would cover that, easy. You need people to trust you with their car, remember.
Something like "Saving for University" or "Help Teens Fight the Recession"
Adults will get a kick out of that.