Federalism, to date, has proved unattractive to the United Kingdom. The United Kingdom is commonly described as a unitary state, whereby governmental power is primarily exercised through a sovereign Parliament at Westminster. The UK may be distinguished from Federal countries, notably the United States or Germany. In federal systems, sovereign power is shared between the federal government and the states. However, the description of the United Kingdom as a unitary state is an oversimplification as there are many instances of devolved, shared and autonomous powers that do not easily fit under a centralized view of the state. These ‘quasi-federal’ elements of the constitution arise through the UK Parliament delegating to regional and local communities a variety of powers and responsibilities through elected local and municipal authorities as well as devolved ‘deals’. Since 1989, powers have been distributed to the four nations of the United Kingdom: England, Scotland, Wales and Northern Ireland through extensive, and increasing, devolved powers (devolution) including a variety of tax-raising powers. There is also a London Assembly with devolved powers. The future of the UK after Brexit is uncertain and there are deep divisions of opinion. England and Wales voted for Brexit while London, Northern Ireland and Scotland voted to remain within the EU. Different constitutional configurations were suggested for the four nations, during the nineteenth century, including federalism, Irish home rule and independence as well as strengthening local government. No exact definition of federalism emerged from the different variations supported at one time or another during this period. Consequently supporters of federalism have struggled to have a single configuration to make their case. Overall federalism was rejected as inconsistent with the orthodoxy of a unitary state formed from an incorporating union centred around a sovereign Parliament. Has the extent of substantial devolved and delegated powers reached a tipping point that places a form of divisible federalism as a way of addressing current concerns and controversies including Brexit? Any formal adoption of federalism would alter the role of the UK Supreme Court as well as future relations with the EU after Brexit. Federalism might provide a mechanism for a changing unitary state to address 21st-century challenges amidst a perceptible shift to a ‘quasi-federal’ state with devolved governments and many shared or delegated powers.