There are several factors which strengthen Cabinet Ministers positions against the Prime Minister.
Ministers run large Departments on a day-to-day basis, dealing with many issues. They have a large civil service with technical expertise and their own political advisers.
They also have overall responsibility for a range of Agencies and Quangos. The Prime Minister can intervene, though in practical terms not on everything, and, although he or she also has a civil service and political advisers, intervening within a Department takes a lot of effort.
The Prime Minister has to spend time coordinating issues that cut across Departments and may not have time to look at many purely Departmental issues.
Ministers are politicians who are often major figures in their Party. They may have a power base in both the the Party in the country and the Parliamentary Party.
They may be hostile to the Prime Minister or rivals for the office. Tony Blair had to appoint Ministers who were supporters of Gordon Brown and this gave Brown power to delay or stop many policies.
Ministers have links to the media and, through their Departments, to interest groups. They can leak information and brief journalists against policies that the Prime Minister wants to pursue. The Department can warn interest groups of a policy detrimental to them that the Prime Minister wants to pursue so that these interest groups can put out adverse publicity against it.
The cabinet is bound by collective responsibility. This means a decision taken in a Cabinet meeting, even if not all members agreed, has to be supported by everyone.
If this is not possible, a combined revolt by the Cabinet can prevent a policy advocated by the Prime Minister. It can also be timed for maximum embarrassment. In 2018, Theresa May confidently claimed she had the ‘full-backing’ of cabinet for her Chequers Agreement. This agreement set out negotiating terms for leaving the European Union. Within two days her Brexit Secretary and Foreign secretary had resigned. They did not do this at the time. Some cynically say this was so they could be driven home in their ministerial cars : )
They can even bring the Prime Minister down as happened to Thatcher in 1990.
Ministers can be sacked but this places them on the back benches of the House of Commons where they have the freedom to say what they want (more or less)
Margaret Thatcher was severely weakened by Chancellor of the Exchequer, Nigel Lawson, and Deputy Prime Minister, Geoffrey Howe’s resignations. They made incredibly powerful resignation speeches in the Commons. The same happened to Tony Blair, when his Foreign Secretary, Robin Cook resigned as a protest to the planned war in Iraq.
