Brexit removed the external support for Devolution. The territorial tensions are made worse by the fact that majorities in Scotland and Northern Ireland opposed it. There have been three responses: United Kingdom as a unitary state; to fragment into its nations; and to seek a Brexit with exceptions. Only Northern Ireland has been granted n exceptions.
"The United Kingdom has presented a model of territorial/ national accommodation which, ... provided a basis for negotiation and adjustment. This rests upon historic understandings of the state as a union... Nation, state and sovereignty were all contested but this rarely became a zero-sum game; even the prospect of Scottish independence in 2014 was eased by the fact that both successor states would remain within the EU. ... Brexit, however, represents an assertion of a territorially-bounded and internally-integrated state which, arguably, the United Kingdom never was.
The EU has never been an actor in the UK constitutional reform or the territorial fall-out from Brexit, except in the case of the Irish border, where it had a stake in protecting its own economic border and in supporting a loyal Member State. Similarly, it has stayed out of territorial conflicts in Spain and elsewhere....
Another lesson from the case concerns differentiated integration.... The EU has permitted Member States to opt out of some aspects of integration and non-Member States to opt in. It has allowed regions of some Member States to opt out, wholly or entirely. The Brexit agreement, with its provisions for Northern Ireland, is the first instance of a region of non-member state having a significant opt-in. ... Brexit has increased centrifugal tendencies within the United Kingdom but they remain a matter for a domestic political arena now dominated by unitarist and centralizing tendencies."
Journal of European Public Policy, Volume 29, 2022 - Issue 4