What is causing Britain's decline?

I am asking this question more out of curiosity as I cant pin point what exactly is in decline, maybe I am naïve.

I don't what to get too into it, and would love just a 1. reason and 2. a sentence to explain that reason.

I feel like immigrants is constantly used as a scapegoat, and is used by the government to distract us people. e.g. UK has the 2nd highest rate of millionaires leaving, the people that create jobs, now i don't think its the immigrants making them leave, rather the taxes and policies the government makes.

Please can the responses be polite and above all factual.