Ministerial ranking

The ministerial ranking or order of precedence in Cabinet refers to the "...pecking order" of senior ministers in the UK government. However, its importance is debated.
The ministerial ranking is said by Peter Hennessy to be decided by the Prime Minister alone. However, in his autobiography David Cameron said that it "...combines seniority of post and the length of time as a cabinet minister to determine the rank of everyone present".
Importance.
Hennessy says that it "...matters more than one thinks in establishing the power of a Prime Minister in relation to his most senior colleagues." 
The Cabinet Manual states that when the Prime Minister is unable to attend Cabinet, or the chair and any deputy chair of a Cabinet committee are absent, the next most senior minister in the ministerial ranking should take the chair.
David Cameron has stated that it was "something we had never bothered with". Additionally, in April 2020, when Boris Johnson was moved into intensive care with COVID-19, a government press released stated that he had asked First Secretary of State Dominic Raab "to deputise for him where necessary", but the ministerial ranking on the parliament.uk website around the time showed Rishi Sunak technically ranking above Raab. 
One constitutional law academic, Rodney Brazier, has suggested that if the Prime Minister were to die suddenly, the monarch could ask the Deputy Prime Minister, or if there was no such person available, the next most senior MP in the ministerial ranking to take temporary charge of the government.
Current ministerial ranking.
The current ministerial ranking, as of April 2021, is as follows: