Before the age of widespread international computer-mediated communication, authorities generally agreed on a rule of starting with one and alternating to the other for quotations within quotations. Many agreeing that starting with single quotation marks was the U.K. rule and starting with double quotation marks was the U.S. rule; with the U.K. switching to double quotation marks for quotations within quotations and the U.S. switching to single.

One U.K. authority that I have from 1985 describes U.K. use of double quotation marks in primary position as "fighting a rearguard action" in the U.K., with only The Times sticking to it. Everyone else used single quotation marks primarily, back then.

However, the influence of CMC has put a lot of pressure on the then U.K. habit. Today, after decades of Usenet, Fidonet, the World Wide Web, et al., the top articles on BBC News (https://www.bbc.co.uk/news/world-europe-66007017) and The Guardian (https://www.theguardian.com/world/2023/jun/24/wagner-troops-...) use double quotation marks, and the pendulum has very much swung the other way, with the U.K. norm of the 1970s now being the rare exception.

However, the rule of switching for quotations within quotations still holds, and headlines quite often still use single quotation marks, even though article bodies will use double ones for the same quotation. This latter is observably the case on the BBC News and Guardian sites right now, to use the same examples.