Just to correct you slightly: that would be leaving with a deal, it's called the WTO deal. And it sounds wonderful. No need to pay £39 billion to EU. Just tell them if THEY want money or a deal, then they can call 999 mwhahaha.
When a couple marries they have both rights and obligations. They both have to contribute to pay all the expenses. They have to agree on that if not they cannot marry.
If they buy a house, each person has part in it. In general (except if agreed differently) they both have to pay and both will receive their part when the house will be sold. In case of a divorce all obligations linked with the marriage (loans and other debts, saving accounts, taxes...) should first be ended, and paid if needed, before the divorce can be final.
One partner cannot say: I am out, you can pay all the bills for which we engaged together. They are both legally bounded to their obligations.
The same is true for membership of the EU. When you enter you have an agreement to sign. With 99.99% certainty nobody on ET ever read this agreement. The EU takes all the time long term engagements that are always approved by the members. These members also approve the amount of money that is paid by each member state, as well the subsidies they would receive. So also their OWN obligations towards the EU. As the UK legally bounded themselves in these long term commitments, they have to fulfill completely their financial obligations. They have to pay their part of the longterm engagements they agreed on.
The UK acts like the person who wants a divorce and wants to dump all the expenses, and engagements and let the others pay. The future engagements have to be calculated and be paid for their part by the UK. They cannot engage, run away and leave the bill with the other members. I think if they will do that they will be prosecuted by the EU.
I wonder how many people on this forum have any clue about the agreement that was signed between the UK and EU when the UK entered. A lot of experts in these matters have problems to solve the is complex situation. But on ET we do not only have experts in trading, we apparently also have experts in EU legislation.
I am not an expert, just read a bit about the situation and the cause of the financial dispute.
All parties envolved will take a hit, there will be no winners except for the lawyers.