The OUP Blog (based in the USA) refers to ‘Sark, United Kingdom’. What’s more, this contribution comes from Ben Keene, described as the editor of the Oxford Atlas of the World.

I thought it was a given that the Channel Islands are not part of the UK. But not only Ben, but many other sites believe otherwise. Take Destination360, which has an interesting style of English and presents both views in one piece:

Although the UK Channel Islands are found off of the coast of Normandy, France, they are in actuality part of the United Kingdom. The UK Channel Islands include the large Guernsey Channel Island, as well as Alderney, Jersey UK and the smaller island of Sark. There are also a number of other smaller islands included in the UK Channel Islands, both inhabited and not inhabited.
The history of Guernsey Channel Island and the rest of the Channel Islands include William the Conqueror, who became the monarch of England in 1066 and was coroneted at Westminster Abbey. In the year 1204, England lost control of the Normandy mainland to the French, and since that time the Guernsey Channel Island and the rest of the Channel Islands have been governed separately from the rest of the United Kingdom, though still considered to be possessions of the United Kingdom. Interestingly, they are not considered to be part of the UK, but rather dependents. … Although Jersey UK is inhabited and has a functioning government, it is not represented in British parliament.

I have taught a lot of people that the Channel Islands are not part of the UK or EU, but are crown dependencies. But perhaps many of them have forgotten this. See the definition of the UK in Wikipedia:

The Crown Dependencies of the Channel Islands and the Isle of Man, formally possessions of the Crown, are not part of the UK but form a federacy with it.

The Independent had an article on Sark last year, titled Lost world: the last days of feudal Sark (yes, apparently it’s a democracy now). You can also read about Clameur de Haro there.