Law? Who cares. There is an obligation of good faith here. What is at stake is a massive reordering of the relations between the UK and the EU, affecting all involved. To treat post-Brexit as a unilateral issue to be handled at the leisure and ordering of Ms. May, which is what this attitude is saying, is profoundly insulting and disrespectful, rude in ways commensurate with the mocking face of a Farage. This is now especially the case once Ms. May has made "out means out" the mantra. I imagine the rest of the EU are to patiently stand by and respectfully await whatever it is offered, and be duly grateful? Bollocks.
Closest thing to this attitude is China calling exchange rates a matter for internal affairs alone, or that it alone decides international boundaries. Putin does that, too. Nice company the UK is keeping now that it has left-without-leaving.