Thats a topic thats been raised many times before by motoring organisations and the motoring public.
The same also applies to not having vehicle tax or disk, with the revenue coming from an increase in fuel prices, which will make the higher mileage user pay their fair share, due to mileage covered. At present the road fund license calculations are a shambles.
for road tax purposes then you have already declared the vehicle to be off the road. Declaring a SORN for insurance purposes strikes me as being a duplication of effort (and cost, and burocracy etc...) for no visible gain. I dont think it will actually help catch uninsured drivers, merely make an attention getting headline.
A database of insured vehicles already exists, and is used by the police via ANPR technology (along with the status of the road tax). If the police want to catch uninsured drivers then they need to get out more, not trawl through databases. Because that wont tell them who the uninsured driver is.