My DH, self employed as a joiner, had never claimed sickness benefit, just carried on working through illnesses,until he knelt on a nail. The rusty nail went behind his knee cap, but typically he pulled it out with pliers and finished the job. The next day his knee had swollen, would not bend, he could neither walk nor drive. Off to the Doctor's, got antibiotics, etc. but sadly it was nearly a month until he could work/earn again. He claimed sickness benefit for the first (and last) time ever but was refused as he was deemed to be still working - sending out invoices for work done previously, completing his VAT return, contacting his customers to cancel jobs etc. Meanwhile, his workshop rent and rates, telephone, etc had to be paid, HP, tax & MOT for his van AND his NI and Tax was still due.
We know many in the building trade whose businesses collapsed following an illness or accident, with the loss of their life's work/savings. We were lucky that we only lost a month's income which meant no summer holiday for us that year. We understood that this was one of the reasons that the self employed paid less NI than employees - the other being that they would get a reduced State pension.