If a qualifying business receives government financial assistance such as staff salary payments or VAT deferment, can an insurer use this against them?

The short answer is no, anybody who suffers a loss has a duty to mitigate that loss. That means a person or a business that is suffering a loss has to do the best they can to minimise it so that they can get back to work as soon and as quickly as possible. Applying for government grants and using the 80% salary furlough scheme, is exactly the sort of thing that is envisaged by mitigating your loss. The insurer cannot use it against you, that is to say they cannot simply deny your claim exists at all.  However, when the panel of solicitors are calculating your losses, they will have to take into account, things that you have paid and you have not paid. You cannot claim for staff wages if you have had 80% of them paid by the government, you can only claim if you have made the 20% top up yourself.