Sone straight answers from the Gov or anyone in The Know would be good.
If things continue as they are and either your destination won’t accept incoming visitors, or is imposing severe restrictions the Jet2 have shown that they have been decisive, even to the extent yesterday of taking the lead and returning plane loads of passengers who were well on their way to their Spanish destinations and turning them round. Jet2 won’t take you, you won’t need to worry about the insurance, Jet2 will offer various alternatives, from a refund to changing dates, to possibly offering you an alternative destination if any exist.