It doesn't surprise me Jet2 are holding those who have booked trips from May onward to their T&Cs. The current FCO advice is against all but essential travel "until at least 15 April due to unprecedented international border closures and other restrictions". I expect this advice will shortly be extended for a further 4 weeks. Worthy of note is the advice is not specifically for health reasons but due to the likelihood of being stranded at the destination.
Inbound cash flow will be nonexistent for travel companies and they will want to hold on to what cash they have for as long as possible. If the FCO extends its advice against travel the tour operators and airlines will have to allow those affected to change their reservations, receive a credit for future travel, or perhaps a refund.
If you do cancel you will lose your deposit. Your travel insurance will not provide cover for a disinclination to travel. However, should FCO advice change (and that advice covers the period you are due to travel) then all options are open to you. If travel restrictions remain in effect for Spain and the Balearics that will also give you a gert out. In the absence of one of your flights being cancelled, a further extension of the FCO exceptional travel advisory that covers the period of intended travel, or the travel restrictions for Spain continuing then there is nothing you can do to force Jet2 to cancel your trip or refund your deposit.
Should you decide to pay the balance, I would pay by a credit card so that you enjoy the additional Consumer Credit Act protection in case things go wrong. Do not pay via PayPal or other payment systems, pay direct to Jet2 on their credit card portal.
I don't have a crystal ball so cannot envisage when travel restrictions might be lifted or when flights may resume. I am receiving emails weekly advising flights booked for business trips are cancelled. All trips since mid-March through to the end of April have been cancelled - some by the airline others because of travel restrictions. At present I have flights booked in May for the USA, Singapore, Sydney and Brisbane. What are my chances of any of those trips being flown as booked? I would say, close to nil.