The United Kingdom as a union state
The United Kingdom is a union of England, Scotland, Wales and Northern Ireland in a single state. The Channel Islands and the Isle of Man, which are internally self-governing dependencies of the Crown, are not part of the UK (on the Crown Dependencies and their constitutional relationship with the UK, see House of Commons Justice Committee, 8th report of 2009–10, HC 56).
It used to be generally thought that the UK has a unitary constitution, like those of France, Italy, Japan, the Netherlands, Sweden and New Zealand, and unlike the federal constitutions of Germany (‘The Federal Republic of Germany’), Switzerland, the US, Australia, Brazil, Canada, India, Nigeria and the Russian Federation. However, it may be that the better view is that the UK has a union constitution that is neither straightforwardly unitary nor systematically federal in character (see Walker, ‘Beyond the unitary conception of the United Kingdom constitution?’  PL 384; this is a strong theme, too, of Iain McLean’s book, What’s Wrong with the British Constitution? (2010)). This, perhaps, is particularly true since the advent of the current devolution arrangements in 1998. That said, however, it should not be thought that all such differences as exist in the government and public law of England, Scotland, Wales and Northern Ireland were created by devolution. A number of differences between English and Welsh law, on the one hand, and Scots law, on the other hand, are several centuries old. Others, while more recent in origin, nonetheless have nothing to do with devolution. Examples include differences in the law pertaining to the Crown, in judicial review proceedings and in the law of remedies (see generally A McHarg and T Mullen (eds), Public Law in Scotland (2006), especially chapters by McHarg, Tierney and Tomkins).