There will be hurdles to successful claims. As noted above, it would take a particular fact pattern, dependent on the precise deal that is ultimately struck with the EU and what that means for regulatory change and/or impact on investments in the UK. There is also a real question over whether Brexit and its consequences could properly be attributed to a sovereign act of the UK. Whilst the triggering of Article 50 specifically was clearly an act of Government, the precise consequences in terms of regulatory, tariff and passporting changes will effectively be imposed from outside the UK. Also BITs protect investment measures rather than trade measures.
In addition, there is the fact that Brexit, whilst not widely forecast, was not a totally fanciful risk: the promise of a referendum was part of the Conservative party’s manifesto in 2015, the referendum was in 2016 and the final outcome of Brexit is unlikely to be known until 2019. Even well before all that, the UK Independence Party (which campaigned for Brexit) had existed since 1993 and saw a significant share of the vote at the European elections in both 2004 (taking 15 percent vote share) and 2014 (when it took 26 percent, the largest share of the vote of any single party). Finally, as we’ve seen in the Phillip Morris case and others, Tribunals are willing to accept an “acceptable margin of change”. In other words, the FET standard is not a guarantee against novel state action, and Tribunal’s accept that states must be free, within some acceptable tolerance, to judge what is in the public interest and to legislate in pursuance of that. How this concept might apply to Brexit remains to be seen. In particular, if Tribunals are willing to tolerate some legislative change in pursuit of a government’s judgment as to what is in the public interest, there is arguably an even greater need for tolerance where (as with Brexit) the public interest in question is the directly democratic expression of the public itself.
What is certain however is that any claims against the UK government for Brexit would be highly controversial – legally and politically.