Eddie, many rogue drivers don't bother with insurance or anything for that matter but they have to be caught actually on the road, now they don't, the car can be checked against MID and seized even if it is off the road.
The sorn mob, presumably have their car off the road but insured. Bikers would definately be insured and very few bother with sorning to save about £35, not worth the agro. In fact most of the whole sorn thing is for classic cars bikes where they are off the road for repairs or restoration etc.