A tax tribunal found that working in a massage parlour counts as self-employment, somewhat to my surprise. (It doesn't pass many of the usual 'are they self-employed or not' tests.)
The guides published annually by the Child Poverty Action Group are by far the best explanations to the mess that's the benefit system. If you're claiming a means-tested benefit, there's an enormous discount. See www.cpag.org.uk for more details.