We had hoped that opening a topic on the forum would encourage helpful and informative discussion.
So, here is some helpful information.
Although geographically in the centre of the British Isles, the Isle of Man is most certainly not part of the United Kingdom. We have our own government, economy, national anthem, flag, stamps, money and more. Indeed, the quickest and most reliable way to upset the indigenous population would be to suggest that it is part of the United Kingdom. We do share the monarchy with the United Kingdom but then so does Australia.
As such it is quite correct to use the term “in” the Isle of Man, for caching or indeed any other activity. Why not browse the many posters and booklets (this can be done online) from the island’s years as a top tourist destination which have titles such as “Happy Holidays in the Isle of Man”?
I am sure a Geocaching visit to, for example, Hawaii would be referred to as “Geocaching in Hawaii” rather than “on” Hawaii. Yes, Hawaii has more than one island with geocaches but then so does the Isle of Man.
We hope this has been helpful and perhaps even encouraged some research before erroneous opinions are posted on the site.