They pay NIC on their employees, but not their driver partners. The driver partners like all contractors in the UK are responsible for paying their own NI. I don't see why the contracting company is responsible for enforcing tax law for its partners?

Self-employed people in the UK pay class 2 and class 4 NIC. Those that are class 2 pay £2.95/wk on earnings of £6,205+ per year. Those that are class 4 pay 9% on annual profits of from £8,424 to £46,350 per year. They pay 2% on profits over £46,350.