A ‘bill’ is not commonly subject to negotiation. It arrives after a customer has contracted for the purchase of goods or services, whose price — with the unique exception of American health care bills, which are more like muggings by gangs on mopeds — has been established in advance. For the average upstanding Briton, a bill is not a starting point, subject to haggling. It is something you pay.
The Lisbon Treaty’s Article 50 makes no mention of paying financial liabilities in order to leave the EU. Once the post–referendum conversation turned immediately to the ‘divorce bill’, the May government’s big mistake from the off was bickering about its size. A better opening strategy would run not ‘How much?’ but ‘What divorce bill?’
First of all, we’re not talking about a marriage, but a membership. The mistaken conflation of a highly emotive institution with a prosaic subscription is germane. The dissolution of a marriage can be fraught, and entails a division of assets often unfair, taking little or no consideration of what income each party has contributed to the relationship. In the UK, many a rightly outraged spouse has seen an under-accomplished ex walk off with half the spoils of his or her successful career — which sets an ominous precedent here.
The cancellation of a membership is more straightforward. You no longer enjoy the benefits of the association; you no longer have to obey the rules of the organisation or pay its dues.
Fiscally, that is straightforward: the UK obviously continues to pay its EU contributions while still a member. Should a further transition period involve UK participation in the single market and customs union beyond the spring of 2018, the UK should clearly pay its dues for such a de facto membership.