Step-by-step explanation:
Because a Constitution is designed to tell the government what it cannot do with regards to the citizenry, and what rights it is not allowed to impinge upon. That’s essential in a Democratic Republic - after all, the people’s representatives are the ones making the decisions on behalf of everyone else. That’s slightly different to the system we have in the UK.
The UK isn’t a Democratic Republic: it’s a Parliamentary Democracy, which invariably means that, though we elect our representatives to a central government (in the same way that the US does with regards to Congress), our representatives are there to reflect our views, and not their own. The system of government itself is designed to be fluid, and change over time. When the public asked for stricter gun legislation in the wake of the Dunblane Massacre, we got it. No muss, no fuss. Even something as serious as the EU ‘Brexit’ mandate was something that we voted on beforehand. If it’d been the US, you’d have been told it was happening, or not happening.
It’s also worth noting that many of the Constitutional documents around the world are patterned on UK political documents that helped to shape the concept of modern democracy. The Magna Carta of 1215 helped to place limits upon the actions of autocratic monarchy and instituting the concepts of ‘due process’, the Case of Proclamations of 1610, which asserted that “the King by his proclamation or other ways cannot change any part of the common law, or statute law, or the customs of the realm”, the act of Habeas Corpus in 1679, the Bill of Rights of 1689…the list goes on.
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