Questions about the validity of insurance policies, when the vehicle does not have a road fund tax disc and/or an MoT certificate, occur quite often here on AB. They usually produce a range of conflicting answers! However, I've just read through my policy, which does require a current MoT certificate for the insurance to remain valid but does not impose any conditions relating to road fund tax.
Since the absence of a tax disc does not relate to the road-worthiness of the vehicle, or to the ability of the policy-holder to drive it, I can see no reason why an insurance policy should be invalidated by a driver's failure to renew the tax disc. My own policy seems to confirm this view.
There was a test case where the High Court ruled that it was unlawful for councils to impose a penalty charge on vehicles parked on their car parks, solely because the vehicles did not have valid tax discs. The Court ruled that the presence, or otherwise, of a tax disc was irrelevant to the contract which existed between the driver and the council. It's likely that a court would similarly rule that the presence, or otherwise, of a tax disc is irrelevant to the contract which exists between a driver and his insurer.
However, as I indicated at the start of this post, I would not be at all surprised to see conflicting views posted here ;-)
Chris