The United Kingdom has a devolution system, according to which different policy powers from the United Kingdom's Parliament have been transferred to assemblies in Cardiff and Belfast, and to the Scottish Parliament in Edinburgh. In addition to the negotiations Britain faces to determine its status after it departs the European Union, the central government must also prepare for the issues that will arise among the United Kingdom's constituent countries.
After the Brexit referendum passed in 2016, Scottish authorities lobbied for the United Kingdom to remain a member of the EU's single market (an area where people, goods, services and capital move freely). But the British government says it will withdraw from the single market to negotiate a free trade agreement with the European Union instead. In response, the Scottish National Party said that such a change in the status quo justifies holding another independence referendum. However, Scotland's referendum cannot happen without authorization from the United Kingdom's Parliament. The British government faces a dilemma: If it continues to reject a referendum, nationalism in Scotland could grow; but if it authorizes a new vote, the result would be impossible to predict.
The letter delivered by the British government formally triggering the Brexit included Northern Ireland as one of the main topics for its upcoming negotiations as well. The Good Friday deal, finalized in 1998, brought decades of sectarian violence to an end in the region. But the political system it created, in which unionist and nationalist forces share power, has also frequently led to inefficient governments, most recently in 2017. If the parties cannot form a government, the British government is legally authorized to take direct control of Northern Ireland, but this is something it wants to avoid. The Brexit process will create frictions that enable political instability will continue.
Like their English peers, a majority of Welsh voters supported leaving the European Union in the 2016 referendum. But while Welsh independence is not an immediate threat, Welsh leaders are looking for ways to benefit from the Brexit process. Wales also expects to gain from whatever concessions London makes to Scotland.
Meanwhile, Brexit poses two risks for Gibraltar. The government is worried that Gibraltar's exports will no longer have tariff-free access to the EU single market. It is also concerned about Spain unilaterally closing the border and isolating the territory. The government of Gibraltar excluded the possibility of leaving the United Kingdom and joining Spain, but it recently held talks with Scotland to jointly seek ways to remain in the single market.
In the end, London is unlikely to give Scotland or Northern Ireland a decisive role in the Brexit negotiations, or to make any other moves that would jeopardize the country's territorial integrity. As a result, in the coming years secessionist pressures could become hard to contain in the United Kingdom.