The thing that has changed is that what was once a cabinet system, with a prime
minister first among others each with their own portfolio responsibilities, has become a presidency, without a directly elected president.
Once upon a time, the relevant ministry would work on a policy and then the minister would present it to the Cabinet for discussion and approval or rejection. Once agreed, a policy was supported by the entire cabinet and if push came to shove, a dissenter would resign.
A classic textbook example was the introduction of the death grant by the post-war Labour govt: when the policy was introduced in the House of Commons (something else that has passed into history) the PM Clement Attlee began by saying he considered this such an important proposal that he had got the permission of the secretary of state to let him announce it. Also from that era, the textbook example of the House of Commons being the first to be told, was when Chancellor of the Exchequer, Hugh Dalton, was forced to resign for a minor leak just before presenting his Budget. The bits that are kept secret in modern Budgets are just there to provide media scoops to ensure attention is paid.
In the last few days we have had several ministers with Brexit responsibilities making policy comments on Brexit-related matters and being contradicted either by the PM or "sources at No 10."
The latest wheeze for making policy announcements seems to be to walk along Downing Street with "confidential" documents in transparent plastic files where they can be accidentally snapped by press photographers. And suddenly, grammar schools are the hot topic.
Back to the thread title, Humpty Dumpty set the trend:
When I use a word, it means just what I choose it to mean, neither more nor less.