I've done a few car boots in the past, before they invented "retro" and "vintage" and the prices went up!
In my experience, it's only profitable if you have atleast one high value item, like a bike or electrical item, for us it was usually a pram - but definitely something you stand a good chance of getting £50 for.
If you just want to get rid of junk, bundle it together as 3 for 2 or bogof, and if you can have a couple of little boxes of small things for 20p a pop -especially kids toys, like happy meal figures or little animals, cars etc, then you'll keep the kids happy and mum and dad are more likely to buy too.
Be ready to drop your prices by 11am, so that you can have an empty boot by lunch!