In Channel Islands, UK coins circulate but they are not meant to be made for that.
As usual, anomalies abound. Before decimalisation, the highest denomination was threepence in Guernsey and "one fourth of a shilling" (also threepence) in Jersey. The rest of the coinage was standard British coinage, but our copper and bronze coins and threepences were also legal tender there. Since decimalisation, both Jersey and Guernsey have their own full decimal circulation sets but still also use UK coinage. Though the main purpose of UK coinage is NOT to circulate in the Channel Islands and IOM, I'm sure that the UK authorities do not mind this. The Channel Islands are said to be in currency union with the UK, but their banknotes and coins are not legal in the UK, so that is not a currency union. During the debate on what currency an independent Scotland could use, the UK Treasury actually said that the IOM and Channel Islands are NOT in currency union with the UK, and that they in fact use currency boards, pegging their own pounds to ours. The Crown Dependencies like to keep their financial affairs opaque and so do not advertise this fact!
See also:
The Channel Islands' limited predecimal range.
In the Isle of Man in the 1970s, Manx, Irish and UK coinage was used, though Irish coinage and notes were not legal tender there. In 1978/9, Ireland broke the link with sterling, so Irish coinage was not accepted on IOM after that. IOM is very popular with Irish tourists - when I went there in 1981, I heard Northern Irish accents everywhere!
In 1984 I visited Bavaria. One day I took the train to Innsbruck, just across the border in Austria. The shops there happily accepted my German coins and notes.
In 1981, I saw shops in my home town of Newcastle pricing goods in kroner, since it is a popular destination for Scandinavians, who come over on the ferry for the weekend. So there are all sorts of interesting "unofficial" numismatic situations.