They've tightened the regulations recently and there now has to be insurance registered against a taxed vehicle or you get £100 fine in the post which I believe is automatic.
"If you have a vehicle that is still taxed but not insured, you could face a penalty. This includes vintage and classic cars, motorbikes and motor homes all vehicles that people sometimes leave uninsured for part of the year. If this applies to you, you need to return the tax disc on a V14 (including nil value discs) and declare SORN at the same time."
The bloke I work for has a fleet policy and it brings him a whole world of pain when ever he goes to the post office to tax a vehicle because they don't have listed reg numbers. He also had an interesting time when the police stopped him in an 'uninsured vehicle' because again reg numbers aren't on the database for fleet policies so they show up as uninsured on the ANPR cameras. He then had to convince them he was first the company manager and then that he actually had a fleet policy (that covers custom built vehicles) before they stopped threatening to impound the vehicle and let him go with only a producer.