Good heavens! I can’t believe that a simple question can cause so much grief. I also cannot believe some of the statements made by insurance organisations (and the police). In particular some of the utter drivel mentioned in the “Park Insurance” article beggars belief: e.g. “Did you know that having something hanging from your rear view mirror can invalidate your insurance?”
In the original question it was stated “I know that car insurance is void if the car is not taxed” and the debate widened to discuss MoT. The plain fact is that the contents of the Road Traffic Act 1988, Section 148 (Avoidance of certain exceptions to policies or securities) Makes clear that insurers cannot repudiate liability for a number of listed reasons, among which is (S148 (20 (b)) “The condition of the vehicle”. If an insurer tried to void a policy because no MoT was in force it is almost certain that a court would rely on this section to rule that the insurer could not avoid liability. The EU Directive to which I referred earlier actually goes further than S148 and places even more restrictions on insurers. Article 13 of this Directive has a paragraph on “exclusion clauses” and there are other paragraphs buried within the directive which cover the topic:
https://eur-lex.europa.eu/legal-content/EN/TXT/PDF/?uri=CELEX:32009L0103&from=EN
Plainly and simply there are only a very small number of circumstances where insurers can deny Third Party liability of make void motor insurance policies. Having no tax is not one of them; having no MoT is not one of them, having something dangling from your rear view mirror is not one of them, using a phone whilst driving is not one of them; speeding is not one of them. Whatever insurers or their agents say in their online blurb is immaterial; they cannot impose conditions which are contrary to the law. The police are the last organisation people should consult for legal advice so their publications should be viewed with similar caution.
Insurers may seek to reduce or refuse payments for insured losses which are not part of compulsory cover but even then they would have to show (in a civil court) that their exception clause was reasonable. I would suggest that having no Tax does not alter their risk or the likelihood of an accident and they would probably lose. But I repeat, they cannot invalidate or reduce payments for Third Party claims on the basis that the vehicle had no tax or MoT.