Currently in the UK, Dominic Cummings is the Prime Minister's Chief Adviser. I am trying to learn about this position, but cannot find much information about it. How long has this position existed within the UK government? And is there a list available of people who have previously had it?
The reason you can't find much information on it is that it isn't an official position. The Institute for Government heavily implies that the title of 'Chief Adviser' seems to be a novel term, stating:
In No.10, special advisers are the prime minister’s most senior advisers, often including his chief-of-staff, director of communications or head of the Policy Unit. Others can be very powerful even if they do not have a specific role. Dominic Cummings is referred to as the prime minister’s ‘chief adviser’ and seems to have a wide but unspecified brief.
Dominic Cummings is a 'special adviser' or Spad. These are very flexible civil service positions that are generally used to give party political advice, and are appointed directly by either No. 10 or the Minister in charge of the department they will work in. They differ from the regular civil service in that they are not required to be politically neutral, and can provide aid with, for example, drafting political speeches.
The position occupied by Cummings is that of the most senior Spad serving in No. 10. This is usually the Downing Street Chief of Staff, a position created in 1997 under Tony Blair, and most memorably held in recent years by the duo of Nick Timothy and Fiona Hill, who would go on to be blamed by many for Theresa May's disastrous 2017 election result. This post is the closest parallel to Cummings' position, and may provide the history of the role that you seek.
Why Cummings hasn't taken up this position is unclear, as clearly he de facto holds the powers of the role, and it is currently vacant. Indeed, after Boris Johnson became Prime Minister in 2019, the Guardian described his appointment as being "in place of a formal chief of staff".
The chief adviser position is a retitleing of the "Chief of Staff" position that had previously existed since Tony Blair's premiership.
Prior to that, there have been various "special advisers" who were political appointments (and not part of the permanent civil service) since the 1960s (specifically under Harold Wilson, who thought that the permanent civil service, with its preponderance of public-school-and-Oxford educated members had a built-in Tory bias). The first such special advisor was probably Marcia Falkender, (Baroness Falkender), who was said to have been the model for the "Dear Lady" in Yes Minister, but was styled "political secretary and head of political office"
Dominic Cummings is the first to have held the specific title "Chief adviser to the Prime Minister.