No, welshlibranr, the compulsory element of insurance required by the Road Traffic Act cannot be invalidated, revoked or made void because the vehicle is untaxed. The only matter which can normally invalidate such cover is where the driver is disqualified from driving or does not hold (or is disqualified from holding or obtaining) a licence to drive. Any other offence committed, whether it concerns the vehicle (such as no MoT or unroadworthiness) or the driver (excess alcohol, for example) or the driving (speeding, careless or dangerous) does not invalidate the compulsory element of insurance. If it did prosecutions for all such offences would be automatically accompanied by a prosecution for No Insurance. A visit to your local magistrates’ court will show that this is not so.
In fact, unless it says otherwise specifically in the policy, such offences would not invalidate the voluntary element of motor insurance (such as Fire, Theft or Accidental Damage to the insured vehicle). Most policies carry a “roadworthiness” stipulation but mine most certainly does not mention cover being invalid if the car is not taxed.