http://www.direct.gov.uk/en/Motoring/OwningAVe hicle/Motorinsurance/DG_067639
It will also be considered that the person was speeding-while-uninsured.
You can get a good lawyer who will no doubt plead that 'he thought he was insured', but you can imagine of the thousands of those prosecuted every year for the same offence, at least half plead the same in their defence.
If someone is seriously injured by the uninsured speeder, insurance will be unforthcoming simply because the eligibility for insurance cover was overlooked. As such, it is no defence.