Why did UK join EEC, importance of EEC, and were those benefits real or did not fulfil the promises?

UK and Brexit

“The United Kingdom  has left the European Union and will once again be free to make its own laws, post the Brexit Transition period. Some argue that the UK’s parliamentary sovereignty was indeed undermined by the EU and that the UK has finally got back its independence. It is likely that the UK will regret Brexit.”
In relation to the above quote is this alleged new found independence real or just a façade?
In your answer refer to relevant court decisions to support your arguments.
LENGTH 1500 words maximum

What Parliamentary sovereignty is?
* You may talk about origins of the idea of Parliamentary sovereignty. Divine Rights vs
* Bill of Rights 1689 and Claim of Rights
* Why did UK join EEC, importance of EEC, and were those benefits real or did not fulfil the promises?
The respective powers of the 3 branches of government
* Legislature
* Executive
* Judiciary
Has joining EU compromised Parliamentary Sovereignty?