Either the figures are wrong or 1/15 drivers are not insured
Given you can't get insurance without valid TAX and MOT, we could assume no TAX = no insurance.
Lets say car tax is £200 (its less but its easier for figures), that 2 million vehicles. There are about 31 Million vehicles in the UK, and about the same amount of drivers.
So that's 1 out of every 15 drivers not insured.