Most people tend to say Hi on their first post, but ho hum..
A lot of the chippys already have places that buy their used oil from them, the days when they would pay YOU to take it away are for the most part long gone, usual price is 20p - 25p a litre, but you might be lucky.
You can use clean oil (£1 a litre at the moment in cash and carry), and yes it IS legal to use on the road (2,500 litres per year if I remember correctly, so beware of this if you intend to sell any).
My Pajero runs on pure oil (I did used to mix but it seems solid enough om pure) although I do add some white spirit just to thin it down a bit and stop the filter and injectors from waxing. Pre-heater went the other day so now I have to pour boiling hot water on the filter to start it in the morning....fine for the rest of the day
