英國擁有世界上歷史最悠久的立憲及代議制政府，但是，英國憲法是一套不成文的、習慣的、非法典化的，且歷經數世紀演變的憲法。它與北美及西歐國家盛行的現代民主成文憲法雖具有某些共同特徵，也僅是部分。英國憲法在十九世紀中葉至二十世紀中葉間獲得高度推崇，近幾十年來卻飽受批評，且在最近遭到大幅度的修改。加入歐洲聯盟，將立法權從西敏寺下放至蘇格蘭、威爾斯與北愛爾蘭的地方立法及行政機關等決定，使得長久以來「君臨國會」的主權面臨挑戰，法治發生重大改變，同時，也破壞了過去行政機關向國會及人民負責的做法。英國憲法已不再像過去那樣令人稱頌，同時尚未發展出一套足以獲得社會各階層以及王國各地區廣泛支持的新憲法。這種情況導致大眾的意見嚴重分歧，對於英國該如何因應加入歐洲聯盟以及西敏寺權力下放至蘇格蘭、威爾斯與北愛爾蘭後引發的種種改變，無法產生共識。

Britain has the oldest constitutional and representative government in the world, but it is an unwritten, customary and un-codified constitution that has evolved over centuries. It possesses some, but not all, of the features now associated with modern democratic written constitutions widespread in North America and Western Europe. While this constitution enjoyed considerable support from the mid-nineteenth through the mid-twentieth century, in more recent decades it has been subject to much criticism and has been altered significantly by major recent developments. Membership of the European Union and the decisions taken to devolve legislative authority away from Westminster to local legislatures and executives in Scotland, Wales and Northern Ireland have together led to challenges to the long-established sovereign authority of the crown-inparliament, made significant changes to the rule of law, and undermined the former ways in which the executive was accountable to parliament and people. The United Kingdom no longer possesses the constitution so admired until recently but has not yet evolved a new constitution capable of winning widespread support from all social strata and all parts of the kingdom. The result is a public sharply divided on how the United Kingdom should respond to the changes wrought by its membership in the European Union and the devolution of some of Westminster’s authority to Scotland, Wales and Northern Ireland.