tbh May is being realistic. There is absolutely nothing the rest of the EU can do to enforce the issue, but I imagine they can make it increasingly uncomfortable with legislative and regulatory changes

keep to the Fen Causeway

It would be a wry twist of fate if the UK has stumbled into a sweet padded spot on the fence, finally!
Conversely equally twisted would be that the markets instability as the UK waffles and wavers wobbles the already fragile balance of EU finance off its Austerity Axis into the Keynesian (Keen for short) into the arms of Corbyn, Podemos, Syriza and MV5*...
Ben venga!

'The history of public debt is full of irony. It rarely follows our ideas of order and justice.' Thomas Piketty