The Convention of Collective Responsibility is a political and therefore non-legally binding rule of the UK Constitution which can be found in both the Ministerial Code and the Cabinet Manual. It stipulates that government ministers, in particular the cabinet, must publicly agree and abide by government decisions (irrespective of their personal views) and that they should not disclose what was said privately amongst government ministers in reaching these decisions. Should a minister be incapable of doing this, they are expected to resign. If they don’t resign, the Prime Minister has the right to dismiss them.
SpAds (such as Cummings) are temporarily appointed Civil Servants who do not represent the Government in any formal sense and therefore not subject to the convention.