It is not the police who decide if a policy is void, it is the insurers i.e. those who stipulate the conditions of their policy.
An insurer can stipulate that they will not pay out where there is no MOT, Road Tax, car is not roadworthy, or whatever they choose, but there is no law which states that all policies will become void because of these legal infractions.
Consider the offence of 'causing death by dangerous driving'. Even a serious offence such as this (maximum 14 years imprisonment) does not automatically invalidate insurance coverage.