[Olog-hai]

I would not advise using the Treaty of Lisbon’s Article 50 for Brexit, because it gives all the advantage to the EU in terms of negotiation leverage. Instead, rebus sic stantibus ought to be invoked. No such thing is coming into the mind of pro-EU Theresa May, though.

[Oatka]

Instead, rebus sic stantibus ought to be invoked.

The EU will claim pacta sunt servanda (promises must be kept).